PURPOSE: To enhance the volumetric efficiency by controlling a by-pass flow passing through an orifice communicating a high pressure side passage and a low pressure side passage by fluid pressure in the high pressure side passage.

CONSTITUTION: A variable delivery hydraulic pump and a hydraulic motor are connected by a pair of high and low pressure passages 23, 24 to form a closed circuit. When fluid pressure in the high pressure side passage 23 is low, fluid in the high pressure side passage 23 is by-passed through an orifice 35 to the low pressure side passage 24. When the fluid pressure increases, the orifice 35 is closed by a stopper 37 against the force of a spring 39. Accordingly, a lowering of the volumetric efficiency due to a by-pass flow can be prevented.