To simplify setting and fate adjustment of an operation current value and reduce the secular change of the set operation current value to stabilize operation by installing a cam body connected to a current adjusting knob through a screw means, and engaging the end of an operation rod of a reverse operation mechanism with the cam surface of the cam body.

A current adjusting knob 42 is rotated to adjust a gap(g) formed between a temperature compensation bimetal 32 connected to an operation rod 31 engaged with a spiral cam surface 44a of a cam body 44 and a differential lever engaged with a moving plate 18 or a differential plate 19. By fixing a stopper to the current adjusting knob 42, adjustment of positioning of the operation rod 31 and the cam body 44 is completed. In order to conduct set current value fine adjustment, a screw driver is inserted into a through hole 42b formed in a current adjusting knob 42, engaged with a screw means 43, and the screw means 43 fixed with a reverse screw is rotated with the screw driver to adjust the position of a cam surface 44e to the operation rod 31 of the cam body 44, and thereby, current setting can be made from the front.