To drive plural hydraulic actuators by a single hydraulic pump by interposing a spool where pressure oil is applied as back pressure to a back chamber between a hydraulic pump and a hydraulic actuator, and selectively disabling a relief valve for keeping the oil pressure in the back chamber to a designated value from conducting relief operation.

In a hydraulic clutch transmission for a running vehicle, pressure oil from a hydraulic pump 2 driven by an engine E is fed to a main transmission selector valve through an oil passage 4 and further introduced to a double speed cylinder 26 of a double speed clutch 31 through an oil passage which diverges from the middle part of the oil passage 4 and is provided with a selector device. The selector device has a solenoid S as an electric actuator, so that a spool 22 is moved in the axial direction against a spring 23 through a push pin 20 by the power to switch a passage. The push pin 20 has the forward end formed as a valve element which is functioned as a relief valve, and is capable of setting back pressure of the spool 22 in co-operation with a spring 21.