To shorten an axial direction length by providing with many cylinders mechanically connected to a first axis, an eccentric cam formed integratedly with a second axis, a pintle selector valve selectively connected to a piston group, and an outside fluid connecting port.

When a mechanical input is applied on a second axis 12, an eccentric cam 52 connected to a flange part 55 by a bolt is rotated through a rotary port axis 14. When an outside hydraulic source is connected to an outside hydraulic source port 81, a suction/delivery port 22 arranged on the bottom part of a cylinder is divided into an advancing piston and a retreating piston group, and they are automatically and selectively connected through a rotary port groove 27 and an axis 14 by the pintle selector valve. As a result, two couples of ellipse ports 26 are functioned as a selector valve in synchronism with the eccentric cam 52, each piston 30 is operated as a differential hydraulic motor when the eccentric cam 52 and the cylinder block 20 are relatively rotated, and is moved in reciprocation with speed in proportion to a feed fluid flow rate, having the function of the differential hydraulic motor. The length can thus be reduced.