To provide an output revolving speed control device simple in structure and excellent in transmission efficiency by integrating axial piston pump mechanism between an input member direct-coupled to a drive source, and an output member interlockingly connected to a driven machine.

When an input member 13 is started along with a crankshaft 10 of an engine, each piston starts reciprocating motion in association with the rotation of a cylinder block 17, and operating oil is sucked into a bore 18 in a suction stroke through a suction port connected to an oil reservoir part in an operating space. This operating oil is discharged to a discharge port 22b in a discharge stroke. The discharge port 22b which is limited closed space, however, is filled with discharge oil in the whole volume in an instant and put in a high pressure state, and side force acting upon a cam plate 20 rotates an output member 15 including the cam plate 20 practically put in a locked state, synchronously with the input member 13, so that auxiliary machines connected to the pulley part 15c by belt transmission is also rotated at a specified rotating ratio to the output member 15.