PURPOSE: To heighten airtightness because an operation shaft connecting the inside and outside of a case is not needed, improve reliability, simplify structure and contrive reduction cost of production because a part forming a corner can be eliminated in explosion-proof construction.

CONSTITUTION: An outside zero point adjustment mechanism is provided with magnetic sensors 3, 4 on a circuit board 2 set in the inside of a case 1 of airtight or explosion-proof construction, a rotation direction and a rotation quantity of a cylindrical member 5 are detected with the magnetic sensors 3, 4 by rotating the cylindrical member 5 having a magnet-mounted part 6 on part of an one-side bottom face in the outside of the case 1 approching the magnetic sensors 3, 4 and zero point output of a device can be adjusted.