PURPOSE: To prevent creep running by providing differential pressure detecting means for hydraulic circuits and a volumetric control means for controlling the displacement of a hydraulic pump/motor on the side such that the pumping action is effected in the direction in which the differential pressure between hydraulic circuits falls in a set range when manipulation is made to set a hydraulic transmission to a neutral position.

CONSTITUTION: If pressure liquid is discharged, due to erroneous manipulation, from a hydraulic pump/motor 1 whose displacement has been to be set to zero by manipulation for setting a hydraulic transmission to a neutral position, a differential pressure is effected between both hydraulic circuits 3, 4. When the differential pressure exceeds a set value, which is detected by a differential pressure detecting means 11, the displacement of the hydraulic pump/motor 1 discharging pressure liquid due to the operation of a volumetric compensating means 12, is controlled in the direction in which the differential pressure falls in a set range. Accordingly, if the set value is set at a value at which a vehicle cannot run, no creep running phenomenon occurs. Further, when the differential pressure between the hydraulic circuit 3, 4 are controlled to be substantially zero, the amount of leaking oil in hydraulic pump motors 1, 2 may be restrained to a minimum.