A hydraulic shift transmission for a vehicle including a hydraulic torque converter for transmitting the engine torque to a road wheel driving system. The hydraulic shift transmission comprises a first speed gear train interposed between the hydraulic torque converter and the road wheel driving system, a hydraulic clutch installed in the first speed gear train, a hydraulic control system for supplying a hydraulic pressure through an oil supply line to the clutch, a relief valve mechanism, provided for the supply line, for releasing the hydraulic pressure while the vehicle speed is lower than preset reference value with the engine idling, an oil return port open relative to the supply line and releasing the hydraulic pressure while opened, a piston member for, when slided in either axial direction thereof, closing the return port, a resilient member normally biassing the piston member in the axial direction, and a hydraulic circuit for, at least either when the vehicle speed becomes higher than the preset reference value or the engine is accelerated, gradually urging the piston member in the axial direction.