PURPOSE: To permit oil hydraulic motor speed change by providing a change-over valve plate for connecting and disconnecting a plurality of communication passages between a distributing valve and a fluid pressure converting section.

CONSTITUTION: A change-over valve plate 18 for connecting and disconnecting communication passages 15b is provided between stationary plates 16 and 17 respectively connected to a distributing valve and a fluid pressure converting section. When the change-over valve plate 18 is held at the illustrated position, the communication passages between the distributing valve and fluid pressure converting section are all connected, and thus the oil hydraulic motor is rotated at low speed to provide high torque. When the change-over valve plate 18 is switched over to its upper position, three communication passages are disconnected to leave only six inter-gear gaps in the fluid pressure converting section for converting the fluid pressure into the rotational force. Therefore, the oil hydraulic motor is rotated at high speed to provide low torque.