PURPOSE: To stable the speed changing operation of a motor for effecting speed control in three stages, by smoothing the action of a plunger for driving a collar for placing an eccentric drum in a medium speed position.

CONSTITUTION: When achangeover value 17 is shifted to a position C, a port B is connected to a pump 18 to supply pressurized oil to a passage 11a. Since a check valve 14 is provided between a piston chamber I and the passage 11a, the pressurized oil flows into a piston chamber H first. Plungers 8B, 8A are pushed out rightward and leftward by the supply of the pressurized oil into the piston chamber H and moved until collars 9A, 9B come into contact with an inside wall. Subsequently, the oil pressure in the piston chamber H rises to open the check valve 14 so that the oil flows into the piston chamber I to move an eccentric piston 6 downward. As a result, the eccentric piston 6 is placed in a medium speed position by the collars 9A, 9B. The wrong action of the plunger 8A, 8B is prevented by the interposition of the check valve 14.