PURPOSE: To apply no impact force to a contact of a microswitch in the ON/ OFF operation of the microswitch to prevent early fault of the microswitch.

CONSTITUTION: When pressure in a primary hydraulic circuit 3 exceeds a set value, a relief valve 1 is opened, pressure oil in the primary hydraulic circuit 3 is increased and flows in an ON/OFF operation mechanism 31. By the hydraulic pressure, a piston 34 which presses a contact 27a of a microswitch 27 is separated from the contact 27a to make the switch off. When the pressure in the primary hydraulic circuit 3 decreases lower than a set value, the relief valve 1 is closed, the hydraulic pressure applied to the piston 34 gradually decreases, the piston 34 is slowly pushed out to press the contact 27a of the microswitch 27, and the switch is made on.