To smoothly change the operation from the OFF state to ON state of a side clutch.

When an operation lever is tilted, a solenoid pin 22 is projected through a solenoid 41, the solenoid pin 22 is projected into the projection part of the cam part 19a of the side clutch, side clutch gears 19L and 19R are moved in a clutch OFF direction and a clutch OFF state is attained. Almost simultaneously with it, a hydraulic cylinder 15 is operated and the side clutch is held in the OFF state by the oil pressure. Also, energizing time to the solenoid 41 is set to a time required until the solenoid pin 22 is mounted on the projection part of the cam 19a for at least one time and the side clutch is turned OFF by a timer 43. Thus, the solenoid pin 22 is surely pulled out at a position where the solenoid pin 22 comes to the recessed part of the cam 19a.