To easily perform maintenance of an electropneumatic converter.

An opening part 16-2 is formed on a wall surface of an explosion-proof case 16, and an electropneumatic converter 3 is fixed to the inside of an explosion proof cover 16-3 to block up this opening 16-2. The explosion-proof cover 16-3 is removably installed on the wall surface of the explosion-proof case 16 by a machine screw 22, and a flowing passage (an air passage) La is formed inside of it. Pipe parts 16-3a and 16-3b are extendedly formed inside the explosion-proof cover 16-3, and these pipe parts 16-3a and 16-3b are inserted into flowing passages L1 and L2 of the explosion-proof case 16. At maintenance time of the electropneumatic converter 3, the electropneumatic converter 3 is pulled out of the explosion-proof case 16 together with the explosion-proof cover 16-3.