exemplary embodiments of the present invention will hereinafter be described with reference to the accompanying drawings . a first embodiment of the present invention will be described with reference to fig1 to 4b . fig1 shows a partial axial cross section of a linear actuator 100 according to the first embodiment of the present invention , taken at a relevant portion of an output shaft . referring to fig1 , the linear actuator 100 according to the first embodiment includes a pair of stator units 6 which in combination form a stator assembly , has a hollow cylindrical shape and each of which includes : a coil bobbin 3 ; a coil 5 which is wound around the coil bobbin 3 ; terminal elements 4 through which electric power is supplied to the coil 5 ; and cylindrical stator yokes which are made of a soft magnetic steel sheet and which are formed by sheet - metal processing , and which each have pole teeth arranged at the inner periphery thereof . the stator assembly composed of the pair of stator units 6 is fixedly sandwiched between a front housing 1 and a rear housing 11 . the linear actuator 100 further includes a rotor which is arranged in the hollow of the stator assembly and which includes : a cylindrical rotor sleeve 9 connected to the front and rear housings 1 and 11 via a pair of bearings 8 , respectively , so as to be rotatable with respect to the stator units 6 of the stator assembly ; and cylindrical ring magnets ( field magnets ) 7 fixedly attached at the outer periphery of the rotor sleeve 9 by , for example , insert fitting . in the hollow of the rotor sleeve 9 , a screw 10 as an output shaft is provided coaxially to the rotor sleeve 9 . a ball screw 10 a is formed at the outer periphery of the screw 10 . retainers 12 , which have a cylindrical ring shape and which are adapted to hold a plurality of balls 13 having a spherical shape , are fixedly attached to the inner periphery of the rotor sleeve 9 by , for example , insert fitting . in the second embodiment , two of the retainers 12 are provided respectively at two places of the screw 10 located apart from each other in the axial direction . the number of the retainers 12 , however , is not limited to two and may be three or more . the balls 13 held by the retainers 12 engage with the ball screw 10 a of the screw 10 . a pin 2 is inserted in the screw 10 so as to be oriented substantially perpendicular to the axial direction of the screw 10 . fig2 shows a radial cross section of the linear actuator 100 of fig1 , taken along line ii - ii . as shown in fig2 , the pin 2 inserted in the screw 10 radially protrudes from the outer periphery of the screw 10 , and on the other hand the front housing 1 has an inner diameter larger than the outer diameter of the screw 10 and at the same time has an inner radius smaller than the dimension which is defined between the axis center of the screw 10 and the top end of the pin 2 radially protruding from the outer periphery of the screw 10 . further , a slit la is arranged at a place of the inner periphery of the front housing 1 located corresponding to the pin 2 . the slit 1 a has a radial cross - sectional geometry substantially analogous to the longitudinal shape of the pin 2 with a slightly larger dimension and has an elongated axial dimension . with the arrangement described above , the pin 2 is prevented from coming off from the slit 1 a and therefore the screw 10 is not allowed to rotate thus moving only forward and backward in the axial direction . the slit la is provided with front and rear stopper members which are located at the front and rear ends of the slit 1 a and which are constituted respectively by portions of the front housing 1 and the rotor sleeve 9 against which the pin 2 , when moving forward and backward , hits , whereby the forward and backward moving ends of the screw 10 are defined . that is to say , the pin 2 as a rotation preventing member which prevents the screw 10 from rotating functions also as a stopper member to define the forward and backward moving ends of the linear movement of the screw 10 . in this connection , a spring , a washer or the like may be provided at the hitting places to thereby prevent the output shaft from getting stuck at the forward and backward moving ends . description will now be made of the retainer 12 shown in fig1 . fig3 perspectively shows the retainer 12 shown in fig1 holding the balls 13 in place . fig4 a and 4b illustrate a portion of the linear actuator 100 of fig1 located in the vicinity of the retainers 12 , wherein fig4 a shows the side view of the screw 10 and the retainers 12 , and fig4 b shows the axial cross section of the screw 10 and the retainers 12 taken along the axis center of the screw 10 . referring to fig3 , a plurality of through - holes 12 a are provided at each of the retainers 12 in such a manner as to be arranged equidistantly from one another in the circumferential direction and located so as to correspond to the ball screw 10 a . thus , the through - holes 12 a are arranged in a spiral manner on the circumference of the retainer 12 . when the balls 13 are put in the through - holes 12 a from the outer periphery of the retainer 12 , the inner tip of the ball 13 protrudes inwardly from the inner periphery of the retainer 12 , and a portion of the ball 13 protruding inwardly is adapted to engage with the ball screw of 10 a of the screw 10 . the retainer 12 holds the balls 13 in place , wherein after the retainer 12 is attached to the rotor sleeve 9 , it does not happen that the balls 13 come off . with the structure that the retainer 12 is fixedly attached to the rotor sleeve 9 , the rotation of the rotor sleeve 9 is transmitted to the screw 10 via a point contact of the ball 13 . in the first embodiment , the retainer 12 is provided with seven of the through - holes 12 a , which corresponds to the number of the balls 13 . the present invention , however , is not limited to this arrangement , wherein it is desirable that the balls 13 support the outer periphery of the ball screw 10 a of the screw 10 without backlash . in this connection , it is not possible to unlimitedly increase the number of the balls 13 in view of the provision of the retainer 12 , so it is preferable to use an odd number of the balls 13 in order to achieve a high precision and a stable load with the least number of the balls 13 . if an even number of the balls 13 are used , when a weight is applied laterally ( perpendicularly to the shaft ), a symmetric position appears , and it can happen that a resultant load is focused on only one of the balls 13 . on the other hand , if an odd number of the balls 13 are used , the load is borne by two or more of the balls 13 and thus can be dispersed . since it is not possible for three of the balls 13 to duly bear the load , and since the load may be focused on one of the balls 13 if four of the balls 13 are used , it is preferable to use at least five of the balls 13 or a larger odd number thereof . according to the first embodiment , since the retainer 12 can be fixedly attached in a unified manner , by means of adhesion , insert fitting or a like method , inside the hollow rotor which includes a magnet 7 as well as the rotor sleeve 9 made of metal ( stainless steel , aluminum , or the like ), the only thing to be done is to set the outer periphery of the magnet 7 coaxial to the inner periphery of the rotor sleeve 9 . consequently , the linear actuator 100 can be assembly with an increased precision and with reduced man hours . it is preferable that the retainer 12 to hold the balls 13 , though not limited in terms of material , be made of metal or abrasion - resistant resin , such as polyacetal ( pom ) or polyphenylene sulphide ( pps ), and also be structured to prevent the balls 13 from falling inside . with such a structure , the assembly can be done easily . a second embodiment of the present invention will be described with reference to fig5 and 6 . fig5 shows a partial axial cross section of a linear actuator 1100 according to the second embodiment of the present invention , taken at a relevant portion of an output shaft . referring to fig5 , the linear actuator 1100 according to the second embodiment is substantially the same as the linear actuator 100 according to the first embodiment shown in fig1 except in that one retainer 112 is provided in place of the two retainers 12 and in that a block 14 is provided in place of the pin 2 . so , identical parts and similar have the same reference numbers as in fig1 , and a detailed description thereof will be omitted . in the hollow of a rotor sleeve 9 , a screw 10 as an output shaft is arranged coaxial to the rotor sleeve 9 . a ball screw 10 a is formed at the outer periphery of the screw 10 . the aforementioned retainer 112 , which has a circular cylindrical shape and which is adapted to hold a plurality of balls 113 having a spherical shape , is fixedly attached to the inner periphery of the rotor sleeve 9 by , for example , insert fitting . the balls 113 held by the retainer 112 engage with a ball screw 10 a of the screw 10 . while each retainer 12 according to the first embodiment shown in fig3 is structured to hold the balls 13 in such a manner that the balls 13 are arranged in one row running around the outer periphery of the screw 10 , the retainer 112 according to the second embodiment is structured to hold the balls 113 in such a manner that the balls 113 are arranged in a plurality of rows running around the outer periphery of the screw 10 . in the embodiment example shown in fig5 , the retainer 112 holds the balls 113 provided with seven of such rows . also , in the second embodiment , the aforementioned block 14 is provided in place of the pin 2 as described above . the block 14 has a polygonal ( quadrangular in the second embodiment ) cross section taken along a direction substantially perpendicular to the axial direction , and is fixedly attached around the screw 10 . fig6 shows a radial cross section of the linear actuator 1100 of fig5 , taken along line vi - vi . referring to fig6 , the block 14 attached around the screw 10 protrudes radially outwardly from the outer periphery of the screw 10 . a front housing 101 , inside which the screw 10 is housed , has a hollow 101 a which has a polygonal radial cross - sectional geometry analogous to and slightly larger than the radial cross section of the block 14 and the wall surface of which is located radially outwardly of the outer periphery of the screw 10 . with the arrangement described above , the block 14 , while prohibited from coming off from the polygonal hollow 101 a , is prevented from moving in the circumferential direction , and therefore the screw 10 having the block 14 fixedly attached therearound is not allowed to rotate and thus allowed only to move forward and backward in the axial direction . the polygonal hollow 101 a is provided with front and rear stopper members which are located at the front and rear ends of the polygonal hollow 101 a and which are constituted respectively by portions of the front housing 101 and the rotor sleeve 9 against which the block 14 , when moving forward and backward , hits , whereby the forward and rearward moving ends of the screw 10 are defined . that is to say , the block 14 as a rotation preventing member which prevents the screw 10 from rotating functions also as stopper members to define the forward and rearward moving ends of the linear movement of the screw 10 . the block 14 may be composed of , for example , two pieces and put together so as to fixedly enclose and grip the small diameter portion of the screw 10 , whereby the assembly can be performed easily while the screw 10 is prevented from rotating and also from axially coming off ( that is to say , the forward and rearward moving ends of the linear movement are defined ). in this connection , a spring , a washer or the like may be provided at the hitting places to thereby prevent the output shaft from getting stuck at the forward and rearward moving ends . a third embodiment of the present invention will be described with reference to fig7 to 10b . fig7 shows a partial axial cross section of a linear actuator 2100 according to the third embodiment of the present invention , taken at a relevant portion of an output shaft . referring to fig7 , the linear actuator 2100 according to the third embodiment is substantially the same as the linear actuator 100 according to the first embodiment shown in fig1 except in that two ribbon retainer 212 are provided in place of the two retainers 12 . so , identical and similar parts have the same reference numbers as in fig1 , and a detailed description thereof will be omitted . in the hollow of a rotor sleeve 9 , a screw 10 as an output shaft is provided coaxially to the rotor sleeve 9 . a ball screw 10 a is formed at the outer periphery of the screw 10 . ribbon retainers 212 adapted to hold a plurality of balls 213 having a spherical shape are fixedly attached to the inner periphery of the rotor sleeve 9 . in the third embodiment , two of the ribbon retainers 212 are provided respectively at two places of the screw 10 located axially apart from each other . the number of the ribbon retainers 212 is not limited to two and may be three or more . the balls 213 held by the ribbon retainers 12 engage with the ball screw 10 a of the screw 10 . a pin 2 is inserted in the screw 10 so as to be oriented substantially perpendicular to the axial direction of the screw 10 . fig8 shows a radial cross section of the linear actuator 2100 of fig7 , taken along line viii - viii . as shown in fig8 , the pin 2 inserted in the screw 10 radially protrudes from the outer periphery of the screw 10 . on the other hand , the inner periphery of a front housing 1 has a larger diameter than the outer periphery of the screw 10 and at the same time has a radius smaller than the dimension which is defined between the axis center of the screw 10 and the top end of the pin 2 radially protruding from the screw 10 . further , a slit 1 a is arranged at a place of the inner periphery of the front housing 1 located corresponding to the pin 2 . the slit 1 a has a radial geometry substantially same as the side shape of the pin 2 with a slightly larger dimension and has an elongated axial dimension . with the arrangement described above , the pin 2 is prevented from running off from the slit 1 a and therefore the screw 10 having the pin 2 inserted therein is not allowed to rotate thus moving only forward and backward in the axial direction . the slit la is provided with front and rear stopper members which are located at the front and rear ends of the slit la and which are constituted respectively by portions of the front housing 1 and the rotor sleeve 9 against which the pin 2 , when moving forward and backward , hits , whereby the forward and rearward moving ends of the screw 10 are defined . that is to say , the pin 2 as a rotation preventing member which prevents the screw 10 from rotating functions also as stopper members to define the forward and rearward moving ends of the linear movement of the screw 10 . in this connection , a spring , a washer or the like may be provided at the hitting places to thereby prevent the output shaft from getting stuck at the forward and rearward moving ends . description will now be made in detail of the ribbon retainer 212 shown in fig7 . fig9 perspectively shows the retainer 212 shown in fig7 holding the balls 213 in place . and , fig1 a and 10b illustrate a portion of the linear actuator 2100 of fig7 located in the vicinity of the ribbon retainers 212 , wherein fig1 a shows the side view of the screw 10 and the ribbon retainers 212 , and fig1 b shows the axial cross section of the screw 10 and the ribbon retainers 212 taken along the axis center of the screw 10 . referring to fig9 , the ribbon retainer 212 is composed of two retainer pieces 214 and 215 , which are formed by , for example , pressing a steel sheet . the retainer piece 214 includes : a plurality of ball holding portions 214 a which are arranged in an equidistant manner and which are each have a curve formed according to the shape of the ball 213 so that the balls 213 are set around the screw 10 equiangularly ; and a plurality of flat portions 214 b which each connect between two adjacent ball holding portions 214 a . in the same way , the retainer piece 215 includes : a plurality of ball holding portions 215 a which are arranged in an equidistant manner so as to oppose the ball holding portions 214 a of the retainer piece 214 and which are each have a curve formed according to the shape of the ball 213 so that the balls 213 are set around the screw 10 equiangularly ; and a plurality of flat portions 215 b which each connect between two adjacent ball holding portions 215 a . and when the two retainer pieces 214 and 215 described above are coupled to each other , the balls 213 can be held in place around the screw 10 equidistantly . the retainer pieces 214 and 215 are put together with the plurality ( seven in the present embodiment ) of balls 213 sandwiched between the respective ball holding portions 214 a and 215 a , and are fixed to each other by , for example , swaging . the balls 213 held by the ribbon retainer 212 are arranged to be located corresponding to the ball screw 10 a of the screw 10 . that is to say , the ribbon retainer 212 is formed spirally in accordance with the spiral of the ball screw 10 a of the screw 10 . in the third embodiment , as shown in fig1 a and 10b , a groove 15 shaped according to the ball 213 is formed at the inner periphery of the rotor sleeve 9 and located corresponding to the balls 213 held by the ribbon retainer 212 . the balls 213 are engaged with the groove 15 , and the ribbon retainer 212 is fixedly attached to the rotor sleeve 9 ( the ribbon retainer 212 may be fixed to the rotor sleeve 9 by adhesive or the like ), and the retainer 212 is caused to rotate in accordance with the rotation of the rotor sleeve 9 . in the third embodiment , the groove 15 is formed at the inner periphery of the rotor sleeve 9 , but the present invention is not limited to this arrangement , and the groove 15 may not be formed at the inner periphery of the rotor sleeve 9 wherein the ribbon retainer 212 to hold the balls 213 may be fixedly attached to the rotor sleeve 9 by adhesion , or the like . the ribbon retainer 212 is adapted to hold the balls 213 in place , and the balls 213 are prevented from coming off from the ribbon retainer 212 . while the ribbon retainer 212 stays fixedly with respect to the rotor sleeve 9 , the rotation of the rotor sleeve 9 is transmitted to the screw 10 via the point contact of the ball 213 . seven of the ball holding portions 214 a and 215 a of the ribbon retainer 212 as well as seven of the balls 213 are used in the third embodiment , but the present invention is not limited to this arrangement , wherein it is desirable to use five or a larger odd number of balls 213 . according to the third embodiment in which the ribbon retainers 212 are employed , the balls 213 are duly held by the ball holding portions 214 a and 215 a and prevented from falling inside or outside . the present invention has been explained with respect to the specific embodiments thereof but is by no means limited thereto . it will be apparent to those skilled in the art that numerous modifications and combinations may be possible without departing from the spirit and scope of the present invention , and also various combinations of the compositions of each embodiment may be included in the present invention .