To provide a power transmission device involving no backlash.

The power transmission device is composed of a driving wheel 2 fixed to a drive shaft 1 coupled with a prime mover and a follower wheel 3 which rotates in abutting to the driving wheel 2 and whereto the torque of the driving wheel 2 is transmitted, wherein the driving wheel 2 is composed of a wheel body 2b fixed to the driving wheel 1 and furnished with a flange 2a at the periphery of one end, a cylinder 2c made of steel fitted on the wheel body 2b slidably with one end abutted to the flange 2a and making swell-out deformation when the other end is pressed, a pressing board 2e sliding on the minor diametric portion 2d of the wheel body 2b and pressing the other end of the cylinder 2c, and an adjusting nut 2g fitted on threads 2f formed on the minor diametric portion 2d of the wheel body 2b to push the board 2e and move and pressing the other end of the cylinder 2c for adjusting the swell-out deformation of the cylinder 2c.