PURPOSE: To relax action speed of a piston while pressurized fluid is supplied to one operation chamber of a cylinder provided with a pair of operation chamber, and to suppress any unexpected action of the piston so as to improve safety.

CONSTITUTION: A four-position switching valve 6 is connected to both operation chambers 1a and 1b of a cylinder body 1, and a position switching action control circuit 35, an action commanding circuit 36, and an auxiliary pressure fluid supply quantity increase control circuit 37 are connected to the switch value 6. When the position switching action control circuit 35 is put in action, any position of the four position switching valve 6 is switched and pressure fluid is supplied to the operation chamber 1a, under a condition where both action chambers 1a and 1b are in their non-pressurized state, the four position switching valve 6 is switched in its action by the auxiliary pressure fluid supply quantity increase control circuit 37 and pressure fluid is temporarily supplied to the other operation chamber 1b so as to suppress any unexpedted action of a piston 2.