PURPOSE: To simply construct a mechanism by equally shortening pressure oil supply passages to a hydraulic clutch type running speed change mechanism and a hydraulically-powered clutch and cutting off the clutch even in such a state that the hydraulically-powered clutch is connected to the speed change mechanism when at a preset speed.

CONSTITUTION: A speed change selector valve V3 for a hydraulic clutch type running speed change mechanism is associated and linked with the first working machine clutch control valve V1 to supply/discharge pressure oil to/from a hydraulically-powered clutch for driving a working machine so that the clutch can be connected/disconnected corresponding to speeds. The hydraulically- powered clutch is mounted in a transmission case M, and the speed change selector valve and the clutch control valve are arranged at the thick portions formed on the different peripheral walls of the transmission case. The second working machine clutch control valve V2 to freely supply/discharge pressure oil to/from the hydraulically-powered clutch is also arranged at the thick portion where the first control valve is arranged.