PURPOSE: To prevent the occurrence of an engaging backlash by providing a spring to push in a direction in which a third gear engaged with first and second gears, being equal in a shape and rotated around the same central axis, and first and second gears approach and a means to change relative displacement in the axial direction between the two gears in a rotation direction.

CONSTITUTION: A fixed gear 1 has a tooth part 1a and a cylinder hub 4 having a rotary shaft hole 1b is fixed to the inner peripheral part 1c of a disc. The inclining groove 5 of the outer periphery of the cylinder hub 4 is engaged with the boss 6 of the inner peripheral surface 2b of a moving gear 2 approximately in the same gear shape as that of the tooth part 1a and a moving gear 2 is moved in a range of a stroke L along the outer periphery of the cylinder hub 4. The moving gear 2 is pressed by the force of a compression spring 7 in such a manner that the tooth part 1a of the fixed gear 1 and the tooth part 2a of the moving gear 2 are prevented from being approximately integrally overlapped. The compression spring 7 is mounted in a compression state in the hole 4a for fixing one end of the cylinder hub 4 by a spring press ring 8 by means of a screw 9. A drive gear 3 is engaged in such a manner to be nipped between the fixed gear 1 and the moving gear 2 and the spring 7 is worked in such a manner to block the rotation of the gear. This constitution prevents the occurrence of an engagement backlash.