To provide a radial type piston motor with a speed reducer capable of making the radial type piston motor with a speed reducer compact, and rotating a case.

A casing 2 having internal teeth 50 on an inner periphery is rotatably borne to a cylinder block 6, and eccentric cam rings 10, 11 having outer teeth 49 internally contacting the internal teeth 50 are pressurized by pistons 12, 13 arranged radially in a radius direction of the cylinder block 6, so that the eccentric cam rings 10, 11 are decentered without rolling along an internal contact face of the casing 2. The eccentric cam rings 10, 11 and the casing 2 constitute a harmonic drive mechanism. An eccentric movement of the eccentric cam rings 10, 11 is picked up by an eccentric cam 9, and rotation of the eccentric cam 9 is transmitted to a swivel member 7 so as to synchronize the rotation of the swivel member 7 with the eccentric movement of the eccentric cam rings 10, 11. Pistons 12, 13 are controlled by relative rotation against the swivel member 7.