To improve the responsiveness of a device by simplifying the structure of an explosion-proof case and shortening a distance between a nozzle and a pneumatic amplifier.

An attaching base 121 is attached to an outer wall of an explosion-proof case 6, and a nozzle flapper mechanism 20 and a pilot relay 26 are attached to the attaching base 121. The nozzle 23 of the nozzle flapper mechanism 20 and the pilot relay 26 are connected through a pneumatic circuit 124 formed in the attaching base 121. The nozzle 23 has an adjusting thread part and is provided movably in free advance/retreat on the attaching base 121 for adjusting a nozzle gap.