it is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention , while eliminating , for purposes of clarity , many other elements which are conventional in this art . those of ordinary skill in the art will recognize that other elements are desirable for implementing the present invention . however , because such elements are well known in the art , and because they do not facilitate a better understanding of the present invention , a discussion of such elements is not provided herein . the present invention will now be described in detail on the basis of exemplary embodiments . fig1 is an explanatory side view showing a schematic structure of a card reader 1 in accordance with an embodiment of the present invention . the card reader 1 in this embodiment is a device for performing reproduction of information recorded on a card 2 and / or recording of information to a card 2 . the card reader 1 includes , as shown in fig1 , a magnetic head 3 for performing reproduction and / or recording of magnetic information , a plurality of carrying rollers 4 for carrying a card 2 in the card reader 1 , a plurality of pad rollers 5 which face the carrying rollers 4 and are urged toward the carrying rollers 4 , a roller drive mechanism 6 for driving the carrying rollers 4 , and a card insertion and ejection part 7 into which a card 2 is inserted and from which the card 2 is ejected . further , a carrying passage 8 where a card 2 is carried is formed in the inside of the card reader 1 . a card 2 in this embodiment is , for example , a rectangular card made of vinyl chloride whose thickness is about 0 . 7 - 0 . 8 mm . a magnetic stripe ( not shown ) in which magnetic information is recorded is formed on a surface of the card 2 . in accordance with an embodiment , an ic chip may be fixed to a surface of the card 2 . the card 2 may be integrated with an antenna for communication in its inside and a printing part on which printing is performed by heat sensitive system may be formed on the surface of the card 2 . further , the card 2 may be a pet ( polyethylene terephthalate ) card whose thickness is about 0 . 18 - 0 . 36 mm , or may be a paper card having a predetermined thickness , or the like . as shown in fig1 , the carrying rollers 4 are disposed on an upper side of the feeding passage 8 . further , the pad rollers 5 are disposed on a lower side of the feeding passage 8 . the pad roller 5 is urged in an upper direction by an urging means not shown so as to be pressed against the carrying roller 4 . the roller drive mechanism 6 includes driven pulleys 10 which are fixed to respective rotation shafts of three carrying rollers 4 , a large driven pulley 11 which is fixed to the rotation shaft of the carrying roller 4 that is disposed at the center so as to be parallel to the driven pulley 10 , a drive motor 12 as a drive source for rotationally driving the carrying rollers 4 , and a drive pulley 13 which is fixed to an output shaft of the drive motor 12 . further , the roller drive mechanism 6 includes a timing belt 14 which is stretched over the drive pulley 13 and the large driven pulley 11 , a timing belt 15 which is stretched over the driven pulleys 10 , and a plurality of tension pulleys 16 for adjusting a tensile force of the timing belt 15 . fig2 ( a ) is a perspective view showing the carrying roller 4 shown in fig1 and fig2 ( b ) is an exploded perspective view showing the carrying roller 4 shown in fig2 ( a ) . fig3 is a side view showing a rubber ring 21 in fig2 ( a ) and 2 ( b ) . fig4 is an enlarged view showing the “ e ” part in fig3 . the carrying roller 4 includes , as shown in fig2 ( a ) and 2 ( b ) , a rubber ring 21 which is made of rubber and formed in a substantially cylindrical shape and which structures an outer peripheral side of the carrying roller 4 , and a core member 22 which is disposed on an inner peripheral side of the rubber ring 21 and holds the rubber ring 21 . in this embodiment , the rubber ring 21 is an outer peripheral member which structures the outer peripheral side of the carrying roller 4 , and the core member 22 is a holding member which holds the rubber ring 21 as the outer peripheral member . an inner peripheral face 21 a of the rubber ring 21 is , as shown in fig3 etc ., formed with a plurality of protruded parts 21 b having a substantially rectangular solid shape which are protruded toward an inner side in a radial direction . in this embodiment , twelve protruded parts 21 b are formed on the inner peripheral face 21 a with an equal angular pitch . further , in this embodiment , each of the protruded parts 21 b is formed over the entire region in the axial direction of the rubber ring 21 . the height “ h ” in the radial direction of the protruded part 21 b ( see fig4 ) is set to be 1 / 3 . 5 or less of the thickness “ t ” in the radial direction of a portion of the rubber ring 21 ( see fig4 ) where the protruded part 21 b is not formed . for example , when the outer diameter of the rubber ring 21 is 20 mm and the width ( width in the direction perpendicular to the paper surface in fig3 ) is 5 mm , the height “ h ” is 0 . 5 mm and the thickness “ t ” is 1 . 75 mm or 3 . 3 mm . in this case , for example , the width “ w ” in the circumferential direction of the protruded part 21 b ( see fig4 ) is 1 mm and the distance “ l ” ( see fig4 ) is about 2 . 5 mm . the core member 22 is , as shown in fig2 ( b ) , is formed in a substantially cylindrical stepped shape , which is provided with a large diameter part 22 b having an outer peripheral face 22 a abutting with the inner peripheral face 21 a of the rubber ring 21 and a small diameter part 22 c having a diameter smaller than the large diameter part 22 b . the core member 22 in this embodiment is formed , for example , of resin . in accordance with an embodiment of the present invention , the core member 22 may be formed of metal such as aluminum . the outer peripheral face 22 a is formed with a plurality of groove parts 22 d which are recessed toward the inner side in the radial direction and extended in the axial direction . each of the protruded parts 21 b is engaged with each of the groove parts 22 d . therefore , the outer peripheral face 22 a is formed with twelve groove parts 22 d with an equal angular pitch . the groove parts 22 d are formed over substantially the entire region in the axial direction of the large diameter part 22 b . further , a depth of the groove part 22 d is set to be the same as the height “ h ” of the protruded part 21 b or slightly larger than the height “ h ” of the protruded part 21 b , and a width of the groove part 22 d is set to be the same as the width “ w ” of the protruded part 21 b or slightly larger than the width “ w ” of protruded part 21 b . an end part of the groove part 22 d in the axial direction of the core member 22 is formed with a flange part 22 e which is formed so as to extend toward an outer side in the radial direction and so as to be abutted with an end part of the rubber ring 21 ( end part in the axial direction ). specifically , as shown in fig2 ( b ) , in one of groove parts 22 d adjacent to each other in the circumferential direction , the flange part 22 e is formed on one end side in the axial direction ( for example , rear side on the paper surface ) and , in the other of the groove parts 22 d adjacent to each other , the flange part 22 e is formed on the other end side in the axial direction ( for example , front side on the paper surface ). in other words , when viewed in the axial direction , the flange parts 22 e adjacent to each other in the circumferential direction are alternately formed on one end side and the other end side in the axial direction . the rubber ring 21 is lightly press - fitted to the large diameter part 22 b of the core member 22 . specifically , the rubber ring 21 is lightly press - fitted to the large diameter part 22 b so that the inner peripheral face 21 a and the outer peripheral face 22 a are abutted with each other and the protruded parts 21 b are engaged with the groove parts 22 d . further , the core member 22 is fixed to the rotation shaft for the carrying roller 4 . in this embodiment , the hardness of the rubber ring 21 single substance when measured by a durometer of type “ a ” specified in jisk6253 is about 65 degrees or about 75 degrees . further , the hardness in the radial direction of the rubber ring 21 of the carrying roller 4 when measured by the durometer of type -“ a ” is in a range of about 67 degrees to 76 degrees . as described above , in this embodiment , a plurality of protruded parts 21 b is formed on the inner peripheral face 21 a of the rubber ring 21 and a plurality of groove parts 22 d which are engaged with the protruded parts 21 b is formed on the outer peripheral face 22 a of the core member 22 . therefore , a slip between the rubber ring 21 and the core member 22 is prevented by the protruded parts 21 b and the groove parts 22 d . accordingly , in this embodiment , an adhering process in which the rubber ring 21 and the core member 22 are adhesively bonded with each other is not required and thus the carrying roller 4 is easily assembled . on the other hand , when the protruded parts 21 b are formed on the inner peripheral face 21 a of the rubber ring 21 , a carrying speed of a card 2 which is carried through the carrying passage 8 is easily varied due to the protruded parts 21 b and thus recording quality and reproduction quality of the card reader 1 may be deteriorated . however , in this embodiment , the height “ h ” in the radial direction of the protruded part 21 b is set to be 1 / 3 . 5 or less of the thickness “ t ” of the rubber ring 21 . therefore , a speed variation amount of a card 2 is restrained to an extent which is near to a speed variation amount of a card 2 in a case that the carrying roller 101 as shown in fig7 ( a ) and 7 ( b ) is used as a reference carrying roller , which is provided with the rubber ring 102 having the smooth inner peripheral face 102 a and made of rubber as a reference outer peripheral member and the core member 103 having the smooth outer peripheral face 103 a , disposed on the inner peripheral side and holding the rubber ring 102 as a reference holding member , or the speed variation amount of the card 2 is restrained to the same extent as the speed variation amount of the card 2 in the case that the carrying roller 101 is used . this effect will be described in detail below on the basis of experiment results . fig5 is a table showing conditions and results when speed variations of a card 2 are measured in the card reader 1 shown in fig1 . conditions of five patterns for the carrying roller are set as shown in fig5 wherein variables are a shape of the inner peripheral face of the rubber ring , a thickness of the rubber ring , a height of the protruded part when the protruded part is formed on the inner peripheral face of the rubber ring , and a hardness in the radial direction of the carrying roller when measured by a durometer of type -“ a ”. the carrying rollers which are set in the conditions 1 through 3 are the carrying roller 4 used in this embodiment , and the carrying roller which is set in the condition 5 is a carrying roller that is similar to the carrying roller 101 as a reference carrying roller . further , the carrying roller which is set in the condition 4 is a carrying roller as a comparison example . in each of the conditions , the outer diameter of the carrying roller is 20 mm . further , when the protruded parts are formed on the inner peripheral face of the rubber ring , twelve protruded parts are formed in a substantially rectangular solid shape on the inner peripheral face of the rubber ring with an equal angular pitch . in addition , in the conditions 1 through 3 where the protruded parts are formed on the inner peripheral face of the rubber ring , the width in the circumferential direction of the protruded part is 1 mm and , in the condition 4 , the width in the circumferential direction of the protruded part is 2 mm . further , the hardness of the rubber ring single substance when measured by a durometer of type -“ a ” is about 75 degrees in the condition 1 and about 65 degrees in the conditions 2 through 5 . as a result of the experiments , a speed variation amount of a card 2 ( ratio of speed variation of a card 2 with respect to the reference carrying speed ) in the card reader 1 using the carrying roller 101 which is set in the condition 5 is 4 - 6 % ( from 4 % to 6 %). further , a speed variation amount of a card 2 in the card reader 1 using the carrying roller 4 which is set in the condition 1 is also 4 - 6 % ( from 4 % to 6 %). in addition , a speed variation amount of a card 2 in the card reader 1 using the carrying roller 4 which is set in the condition 2 is 5 - 8 % ( from 5 % to 8 %), and a speed variation amount of a card 2 in the card reader 1 using the carrying roller 4 set in the condition 3 is 5 - 7 % ( from 5 % to 7 %). on the other hand , a speed variation amount of a card 2 in the card reader 1 using the carrying roller which is set in the condition 4 is 7 - 14 % ( from 7 % to 14 %), in which a thickness of the rubber ring ( thickness in the radial direction of the portion where the protruded part is not formed ) is 1 . 75 mm , the height of the protruded part is 1 mm and thus the height in the radial direction of the protruded part is larger than 1 / 3 . 5 of the thickness of the rubber ring . as described above , when the shape of the carrying roller 4 is set so that the height “ h ” in the radial direction of the protruded part 21 b is 1 / 3 . 5 or less of the thickness “ t ” of the rubber ring 21 , a speed variation amount of a card 2 is restrained to an extent which is near to a speed variation amount of a card 2 when the carrying roller 101 is used , or restrained to the same extent as the speed variation amount of a card 2 when the carrying roller 101 is used . in other words , in this embodiment , even when a plurality of the protruded parts 21 b is formed on the inner peripheral face 21 a of the rubber ring 21 , a speed variation amount of a card 2 is restrained to an extent which is near to a speed variation amount of a card 2 in a case that the carrying roller 101 in which its outer peripheral side is structured by using the rubber ring 102 whose inner peripheral face 102 a is smooth is used or , the speed variation amount of the card 2 is restrained to the same extent as the speed variation amount of the card 2 in the case that the carrying roller 101 is used . as a result , in this embodiment , recording quality and reproduction quality of the card reader 1 can be secured . in this embodiment , as understood from the experiment results , the speed variation amount of a card 2 is further restrained effectively when the carrying roller 4 set in the condition 1 is used , in which the height “ h ” of the protruded part 21 b is set in ⅙ or less of the thickness “ t ” of the rubber ring 21 and , in addition , in a case that the hardness in the radial direction of the carrying roller 101 as shown in fig7 ( a ) and 7 ( b ) when measured by a durometer of type -“ a ” is set to be the reference hardness , the hardness in the radial direction of the carrying roller 4 is set to be substantially equivalent to the reference hardness . in other words , a speed variation amount of a card 2 in the card reader 1 using the carrying roller 4 , which is set in the condition 2 in which the thickness “ t ” of the rubber ring 21 is 1 . 75 mm , the height “ h ” of the protruded part 21 b is larger than ⅙ of the thickness “ t ” of the rubber ring 21 and , in addition , the hardness in the radial direction of the carrying roller 4 when measured by a durometer of type -“ a ” is slightly lower than that in the condition 5 , is slightly larger than the speed variation amount of a card 2 in the card reader 1 using the carrying roller 101 which is set in the condition 5 . further , a speed variation amount of a card 2 in the card reader 1 using the carrying roller 4 , which is set in the condition 3 in which the thickness “ t ” of the rubber ring 21 is 3 . 3 mm , the height “ h ” of the protruded part 21 b is less than ⅙ of the thickness of the rubber ring 21 , but the hardness in the radial direction of the carrying roller 4 when measured by a durometer of type -“ a ” is lower than that in the condition 5 , is slightly larger than the speed variation amount of a card 2 in the card reader 1 using the carrying roller 101 which is set in the condition 5 . on the other hand , a speed variation amount of a card 2 in the card reader 1 using the carrying roller 4 , which is set in the condition 1 in which the thickness “ t ” of the rubber ring 21 is 3 . 3 mm , the height “ h ” of the protruded part 21 b is less than ⅙ of the thickness “ t ” of the rubber ring 21 and , in addition , the hardness in the radial direction of the carrying roller 4 when measured by a durometer of type -“ a ” is substantially equivalent to that in the condition 5 , is 4 - 6 %, i . e ., the same extent to the speed variation amount of a card 2 in the card reader 1 using the carrying roller 101 which is set in the condition 5 . as described above , when the carrying roller 4 is formed so that the height “ h ” in the radial direction of the protruded part 21 b is set to be ⅙ or less than the thickness “ t ” of the rubber ring 21 and , in addition , when the hardness in the radial direction of the carrying roller 4 when measured by a durometer of type -“ a ” is set to be substantially equivalent to the reference hardness which is the hardness in the radial direction of the carrying roller 101 as the reference carrying roller when measured by a durometer of type -“ a ”, a speed variation amount of a card 2 is restrained to the same extent as the speed variation amount of a card 2 when the carrying roller 101 is used . as a result , recording quality and reproduction quality with the same extent as the conventional example can be secured in the card reader 1 . in this embodiment , for example , the thickness “ t ” of the rubber ring 21 is 1 . 75 mm or 3 . 3 mm and the distance “ l ” in the circumferential direction between the protruded parts 21 b is about 2 . 5 mm . however , it is preferable that the distance “ l ” is smaller than the thickness “ t ”. when the distance “ l ” is smaller than the thickness “ t ”, a slip between the rubber ring 21 and the core member 22 is prevented further surely by the protruded parts 21 b and the groove parts 22 d . in other words , according to the experiments of the present inventors , in a case that the outer diameter of the rubber ring 21 is 20 mm , its width is 5 mm , the height “ h ” of the protruded part 21 b is 0 . 5 mm , the thickness “ t ” of the rubber ring 21 is 3 . 3 mm , and the width “ w ” of the protruded part 21 b is 1 mm , when the distance “ l ” between the protruded parts 21 b is set to be 6 mm ( in other words , when six protruded parts 21 b are formed on the inner peripheral face 21 a with an equal angular pitch ), a slip occurs between the rubber ring 21 and the core member 22 . on the other hand , in a case that the outer diameter of the rubber ring 21 is 20 mm , its width is 5 mm , the height “ h ” of the protruded part 21 b is 0 . 5 mm , the thickness “ t ” of the rubber ring 21 is 3 . 3 mm , and the width “ w ” of the protruded part 21 b is 1 mm , when the distance “ l ” between the protruded parts 21 b is set to be 2 . 5 mm ( in other words , when twelve protruded parts 21 b are formed on the inner peripheral face 21 a with an equal angular pitch ), a slip does not occur between the rubber ring 21 and the core member 22 . therefore , a slip between the rubber ring 21 and the core member 22 is prevented further surely through the protruded parts 21 b and the groove parts 22 d by means of that the distance “ l ” between the protruded parts 21 b is set to be smaller than the thickness “ t ” of the rubber ring 21 . in this embodiment , the end part in the axial direction of the groove part 22 d is formed with the flange part 22 e which is abutted with the end part in the axial direction of the rubber ring 21 . therefore , displacement of the rubber ring 21 from the core member 22 in the axial direction is prevented . accordingly , even when the rubber ring 21 and the core member 22 are not adhesively bonded to each other , reproduction quality and recording quality of magnetic information can be secured . especially , in this embodiment , when viewed in the axial direction , the flange parts 22 e adjacent to each other in the circumferential direction are alternately formed on one end side and on the other end side in the axial direction . therefore , when the core member 22 is manufactured by using a die , the structure of the die can be simplified and a manufacturing cost of the core member 22 can be reduced . in a case that a friction coefficient of the surface of a card 2 has been lowered , the card 2 is further adequately carried in the card reader 1 using the carrying roller 4 which is set in the condition 1 in comparison with the card reader 1 using the carrying roller 101 which is set in the condition 5 . the effect will be described below on the basis of the experiment results . fig6 ( a ) and 6 ( b ) are tables showing experiment results in which a card 2 is carried in the card reader 1 shown in fig1 in a state that a friction coefficient of the surface of the card 2 is lowered . an experimenter performed the following experiment 100 times ; a card 2 where a friction coefficient of its surface is lowered is inserted into the card reader 1 from the card insertion and ejection part 7 and then the experimenter checks whether the inserted card 2 is carried in the card reader 1 and ejected from the card insertion and ejection part 7 or not . further , in this experiment , when the inserted card 2 was not ejected ten times continuously ( in other words , a card jam where a card 2 was jammed in the card reader 1 occurred ten times continuously ), the experiment result was determined as “ ng ” and the experiment was ended . further , in the experiment , a friction coefficient of the surface of a card 2 was lowered as follows . in other words , the experimenter wets his or her fingers holding the card 2 with water before inserting the card 2 into the card reader 1 once in five times , or the experimenter puts his or her fingers holding the card 2 with hand cream before inserting the card 2 into the card reader 1 once in twenty times , or the experimenter puts his or her fingers holding the card 2 with hand cream before inserting the card 2 into the card reader 1 once in ten times . in addition , two types , i . e ., the model “ x ” and the model “ y ” are used as the card reader 1 in this experiment . the model “ x ” and the model “ y ” are different from each other in only a width of the magnetic head 3 in a short widthwise direction of the card 2 ( perpendicular direction to the paper surface of fig1 ) and other portions are structured substantially similarly . specifically , a width of the magnetic head 3 in the model “ x ” is about ½ of a width of the magnetic head 3 in the model “ y ”. further , in the model “ x ” and the model “ y ”, the carrying rollers 4 are disposed at a substantially center position in the short widthwise direction of the card 2 . in addition , in the model “ x ” and the model “ y ”, a pad roller is disposed so as to face the magnetic head 3 . as results of the experiments , as shown in fig6 ( a ) and 6 ( b ) , in the card reader 1 using the carrying roller 4 which is set in the condition 1 , in both of the model “ x ” and the model “ y ” and , even when a friction coefficient of the surface of a card 2 is lowered with the use of water and hand cream , a situation that the inserted card 2 is not ejected ten times continuously did not occur . on the other hand , in the card reader 1 using the carrying roller 101 which is set in the condition 5 , as shown in fig6 ( a ), in the model “ x ”, when a friction coefficient of the surface of a card 2 is lowered by hand cream , a situation occurred that the inserted card 2 is not ejected ten times continuously . further , in the card reader 1 using the carrying roller 101 which is set in the condition 5 , as shown in fig6 ( b ), in the model “ y ”, in a case that the experimenter puts his or her fingers holding the card 2 with hand cream before inserting the card 2 into the card reader 1 once in ten times , a situation occurred that the inserted card 2 is not ejected ten times continuously . as described above , in a case that a friction coefficient of the surface of a card 2 has been lowered , the card 2 is adequately carried in the card reader 1 using the carrying roller 4 which is set in the condition 1 in comparison with the card reader 1 using the carrying roller 101 which is set in the condition 5 . this may be because that the thickness of the rubber ring 21 of the carrying roller 4 which is set in the condition 1 is thicker than the thickness of the rubber ring 102 of the carrying roller 101 which is set in the condition 5 and thus the deformed quantity of the rubber ring 21 is larger than the deformed quantity of the rubber ring 102 when abutted with the card 2 and , as a result , the card 2 can be adequately carried in the card reader 1 using the carrying roller 4 which is set in the condition 1 . although the present invention has been shown and described with reference to specific embodiments , various changes and modifications will be apparent to those skilled in the art from the teachings herein . in the embodiment described above , twelve protruded parts 21 b are formed on the inner peripheral face 21 a of the rubber ring 21 with an equal angular pitch . however , the number of the protruded parts 21 b formed on the inner peripheral face 21 a may be a plural number except twelve . further , the protruded parts 21 b may be formed with an unequal angular pitch . further , each of the dimensions of the outer diameter and the width of the rubber ring 21 , the height “ h ” of the protruded part 21 b , the thickness “ t ” of the rubber ring 21 , the width “ w ” in the circumferential direction of the protruded part 21 b , and the distance “ l ” between the protruded parts 21 b shown in the embodiment described above is only an example and thus the present invention is not limited to the dimensions in the embodiment described above . in the embodiment described above , when viewed in the axial direction , the flange parts 22 e which are adjacent to each other in the circumferential direction are alternately formed on one end side and the other end side in the axial direction . however , the present invention is not limited to this embodiment . for example , the flange parts 22 e may be formed on both end sides of the groove part 22 d in the axial direction . while this invention has been described in conjunction with the specific embodiments outlined above , it is evident that many alternatives , modifications , and variations will be apparent to those skilled in the art . accordingly , the preferred embodiments of the invention as set forth above are intended to be illustrative , not limiting . various changes may be made without departing from the spirit and scope of the inventions as defined in the following claims .