To facilitate the assembly and adjustment of a feedback mechanism for improving their working ability.

The peripheral wall of an explosion-proof case 6 is provided with a circular outer wall 81 being protruded outward and a circular inner wall 82 facing in the opposite direction to the inside of the outer wall 81, and a cylindrical sensor storing section 80 opened outward is formed of these inner and outer walls 81, 82. A sensor 11 which detects the turning angle of a feedback lever 8 and converts it into an electric signal is fitted in the sensor storing section 80 with a fixed explosion-proof clearance (d) and its depth L which are kept from the outside of the explosion-proof case 6 and covered with a covering member 84. The rotary shaft 12 of the sensor 11 protrudes outside the cover member 84, and the feedback lever 8 is attached to its protrusive end.