Abstract: An output shaft is constituted by a plurality of rotary shafts which are disposed on the same axial line so as to be rotatable relative to each other. A cam ring which fixedly encloses one of said rotary shafts has a cam surface with radial projections and recessions on an inner peripheral surface of the cam ring. A cylinder block which is mounted on said one of the rotary shafts has formed therein a plurality of cylinders for containing in each thereof a piston which is radially moved by sliding contact with the cam surface. A sun gear is mounted on said one of the rotary shafts. Planetary gears are mounted on remaining at least one of the rotary shafts so as to be rotatable in gearing mesh with the sun gear and an internal gear which is fixedly mounted around the sun gear.