fig1 is a conceptual drawing of a power transmission device of the embodiment of the invention , and fig2 is a partial view of an intermittent gear . in this description , a gear provided with teeth on the whole circumference is referred to as a whole gear for the purpose of discriminating it from an intermittent gear . in addition , the term “ an intermittent gear ” implies , as generally used , a gear having a toothless portion where no tooth is formed . in fig1 , the numeral 1 denotes a driving gear made up of a whole gear , 2 denotes a first follower gear made up of an intermittent gear component 2 a and a whole gear component 2 b , and 3 denotes a second follower gear made up of a whole gear . the drive gear 1 meshes with the intermittent gear component 2 a of the first follower gear 2 , and the whole gear component 2 b of the first follower gear 2 meshes with the second follower gear 3 . as shown by the arrow marks , power is transmitted from the drive gear 1 to the first follower gear 2 , and from the first follower gear 2 to the second follower gear 3 . the rotation of the driving gear 1 which is continuously rotating is transmitted to the intermittent gear component 2 a , and the first follower gear 2 makes a rotation of a definite angle . after the rotation of said definite angle , at the step when the first follower gear has rotated to a position where the toothless portion 2 c is at the mesh position with the driving gear , the transmission of power from the driving gear 1 to the first follower gear 2 is intercepted . because the second follower gear 3 meshes with the whole gear component 2 b of the first follower gear 2 , the second follower gear 3 rotates following the rotation of the first follower gear 2 , and stops in compliance with its stop . accordingly , to the second follower gear 3 , the driving force of the driving gear 1 for the rotation of the definite angle is transmitted . the mechanism of power transmission using an intermittent gear explained above is well known . in the invention , as shown in the drawings , the driving gear 1 , the intermittent gear component 2 a , the whole gear component 2 b , and the second follower gear each is made up of a helical gear . in a power transmission device having a structure as described above , as shown in fig2 , in the process of operation moving from a non - mesh state of the toothless portion 2 c to a mesh state , the teeth 21 , 22 , and 23 start meshing at the leading portions 21 a , 22 a , and 23 a facing the toothless portion 2 c ; that is , the whole tooth trace of one tooth is not simultaneously brought into mesh , as is done in a case where an intermittent gear made up of a spur gear is used . accordingly , the shock at the time the operation moves from the state of no meshing of the toothless portion 2 c to the state of meshing is eased extremely satisfactorily . in addition , in fig2 , the solid lines represent the crests of the teeth and the broken lines represent the bottoms of the teeth . as the result , the generation of shock sounds and the breakage of teeth are sufficiently prevented . on top of it , force acting from the teeth of the drive gear 1 to the teeth of the first follower gear 2 is not directed perpendicularly to the tooth trace . that is , the force acting on the teeth of the intermittent gear component 2 a of the first follower gear 2 is decomposed into a component perpendicular to the tooth trace and a component parallel to the tooth trace . therefore , breaking force against teeth is effectively weakened , and even in a case where the driving torque is large , the breakage of teeth is prevented . fig3 shows a concrete example of a power transmission device of the embodiment of the invention . as already explained , the driving gear 1 meshes with the intermittent gear component 2 a of the first follower gear 2 , and the second follower gear 3 meshes with the whole gear component 2 b of the first follower gear 2 . in the first follower gear 2 , there are provided an engagement part 2 d having a step portion 2 da and a pin 2 e . an engaging member 4 making up a control means engages with the engagement part 2 d . further , to the pin 2 e , one end of a tension - type spring 5 as an actuating means whose another end is fixed at a fixed position is hooked . the engaging member 4 is supported rotatably around a shaft 4 a , and is rotated clockwise , driven by a solenoid 6 that makes up the control means . in the initial state shown in fig2 where the driving gear 1 is continuously rotating in the arrow mark direction , but the power of the driving gear 1 is not transmitted to the first follower gear due to the toothless portion 2 c facing the driving gear 1 , when an actuation signal is inputted to the solenoid 6 making up the control means , the engaging member 4 is rotated clockwise around the shaft 4 a , and the end portion becomes out of engagement with the step portion 2 da . as the result , the first follower gear 2 is rotated counter - clockwise by the driving force of the spring 5 , which makes the driving gear 1 mesh with the intermittent gear component 2 a , to rotate the first follower gear 2 . at the step when the first follower gear has rotated up to a position of an angle where the toothless portion 2 c faces the driving gear 1 , the power transmission of the driving gear 1 is intercepted , the end portion of the engaging member 4 engages with the step portion 2 da , and the first follower gear returns to the initial state shown in fig3 . that is , the first follower gear 2 and the second follower gear 3 stop in the initial state by an actuation signal after the rotation of the definite angle . accordingly , every time an actuation signal is inputted to the solenoid 6 , the first follower gear 2 and the second follower gear 3 make the rotation of the definite angle . fig4 shows an example of a sheet feeding apparatus of the embodiment of the invention having the power transmission device shown in fig3 . in fig4 , the numeral 10 denotes a sheet feed tray , and sheets d are stacked in the sheet feed tray 10 with their leading edges regulated by a regulation member 11 . the numeral 12 denotes a sheet feed roller , and as shown in the drawing , it is formed to have a half - moon cross - sectional shape with a part of a whole roller cut off . the numeral 13 denotes a conveyance roller , which rotates as shown by the arrow mark to convey a sheet d . the numeral 14 denotes a well - known separation roller having a torque limiter built in ( not shown in the drawing ). thus , a separation and conveyance means for separating and conveying one sheet out of the stack of sheets d is made up of the conveyance roller 13 and the separation roller 14 . the shaft 12 a of the sheet feed roller 12 is the rotary shaft driven by the second follower gear 3 in fig2 . by the rotation of the shaft 12 a , the sheet feed roller 12 makes one rotation as shown by the arrow mark , to feed out a sheet d from the uppermost position of the stack . the fed sheet d is conveyed by the conveyance roller 13 , and if multiple - sheet feeding takes place , a single sheet on top is separated by the separation action of the separation roller 14 . thus , the single sheet is conveyed by the separation and conveyance means . fig5 shows another example of a sheet feeding apparatus of the embodiment of the invention . in fig5 , a sheet feed roller 22 is coupled to a conveyance roller 23 by a coupling plate 25 , while it receives a power transmitted from the conveyance roller 23 through a belt 24 , and is driven by the conveyance roller 23 . the conveyance roller 23 and a separation roller 24 having a torque limiter built in as the separation roller 14 shown in fig4 make up a separation and conveyance means . in addition , the conveyance roller 23 is fitted with a one - way clutch ( not shown in the drawing ), and rotates freely in a case where a driving force is applied by a sheet d in the direction shown by the arrow mark . at one end of the coupling plate 25 , there is provided an arm 26 , which engages with a plunger 28 of a solenoid 27 . further , the shaft 23 a of the conveyance roller 23 is the driving shaft , and is driven by the second follower gear 3 shown in fig1 and fig2 . the state shown by the solid line represents a standby state , and the sheet feed roller 22 is not in contact with sheets d . when a sheet feed start signal is inputted to the solenoid 27 , the plunger 28 of the solenoid 27 ascends , the sheet feed roller 25 descends up to the position indicated by the broken line owing to gravity , to become in contact with a sheet d , and presses the sheet d with a constant pressure . simultaneously with the descending of the sheet feed roller 22 , the shaft 23 a rotates , to rotate the conveyance roller 23 and the sheet feed roller 22 , and a sheet d is fed out . at the point of time when the leading edge of the sheet d is conveyed out by a specified length , as explained before , the driving of the shaft 23 a stops . at this step of stop , the leading edge of the sheet d is conveyed by a pair of conveyance roller at the next stage ( not shown in the drawing ). accordingly , even if the shaft 23 a stops , the sheet d is conveyed by the pair of conveyance rollers at the next stage . fig6 ( a ) and fig6 ( b ) each shows another example of the first follower gear . in the example shown in fig6 ( a ), a toothless portion 2 c is formed with border lines inclined by an angle θ against the direction of the axis of rotation as shown by 2 f . in the example shown in the drawing , the angle of inclination θ is determined to be a value of a little smaller than the angle of the tooth trace of the helical gear against the axis of rotation α . the example shown in fig6 ( b ) is a case where the toothless portion is provided with the angle of inclination θ of its border lines 2 f made to be of reverse sign in terms of positive or negative to the angle of the tooth trace against the axis of rotation α . by making the angle θ have a suitable value in fig6 ( a ) and fig6 ( b ), it is possible to make minimum the shock sounds and the wear and breakage of the teeth . fig7 is a cross - sectional view along the line vii - vii in fig2 and shows an example of the shape of tooth . in fig7 , the tooth 22 is formed in a way such that its crest has a shape of a curve that gently rises in the neighborhood 22 a of the border line 2 f between the toothless portion 2 c and toothed portion of the intermittent gear portion 2 a . because the meshing with the driving gear proceeds in a manner as shown by the arrow mark w , the shock at the start of meshing is eased further as compared with the tooth 22 that is formed in such that its crest has a shape of a curve that sharply rises in the neighborhood 22 a . as the result , generation of shock sounds is prevented satisfactorily , and also wear and breakage of the teeth is prevented . further , for example , it becomes possible to make a power transmission device using a small gear with a module of 0 . 6 or under . by an invention described in any one of the structures 1 to 5 , it is possible to provide a power transmission device using an intermittent gear having a high durability , wherein generation of shock sounds and wear and breakage of its teeth are prevented without being accompanied by the increase of cost . further , a power transmission device using a small gear is actualized , and on top of it , it becomes possible to transmit a driving force of a large torque .