PURPOSE: To obtain a speed change mechanism of a swash plate type hydraulic motor which can vary the rotational speed of a hydraulic motor during its rotation by means of swash plates having different inclination angles while adopting a swash plate changeover piston and a control valve.

CONSTITUTION: A control valve is composed of a pressure line 13, a pilot line 14, a compression spring 16, etc. When hydraulic pressure is supplied to a connection port, fluid in the port is shifted into a supply pressure line 13. Pressure is applied to the inside of the pilot line 14 by pressure application of a two-stage speed control pilot valve. A spool 15 is moved rightward, and the pressure in the supply line 13 is transmitted into a cylinder of a swash plate changeover piston 3. A swash plate 1 is pushed to bring a second tapered surface 4b into contact with a wall surface of a housing 17 and changed over to the second speed. With such a setup, exhaust capacity can be varied during the rotation of a hydraulic motor for varying rotation of a hydraulic motor with a constant amount of oil.