To prevent the vibration of a trip rod and the stall of a motor by making a pneumatic piston to a piston with step having a small diameter part always pressurized in oneside bias direction and a big diameter part alternately pressurized in an opposite bias direction.

A main valve is composed of a spool, valve block, valve plate 3, power piston 4, quick delivery valve and housing. A pilot valve is composed of a pilot piston, push rod 8 and actuator pistons 9a, 9b. Both valves exist in the same cave 12 pressurized by a supply air. The power piston and pilot piston are a differential piston. When the pilot signal does not exist in the chambers 10, 11, the air pressure acting to the small diameter part of the piston pushes the piston to the left. When the pilot signal exists in the chambers 10, 11, the piston is pushed to the right.