To facilitate the assembly works and electrical connecting works of a first and a second printed wiring boards into a case to improve an assembly work ability.

The first and the second printed wiring boards 65A, 65B are attached to the back of a terminal board 37 at required spaces and electrically connected with a pin 154 and a connector 155. The terminal board 37 to which these printed wiring boards are attached is fittedly inserted in an explosion-proof case 6 and fixed to a stepped part 141 with a screw 140 and a clip 148. The exciting coil of an electropneumatic converter and the sensor of a feedback mechanism are connected to the second printed wiring board 65B to send out a feedback signal to the first printed wiring board 65A.