PURPOSE: To suppress the multiplication of an error and easily correct the proportional value of a product at the portion of a potentiometer by providing the second operating plate in parallel with the first operating plate at the preset temperature, providing an gap adjusting screw on an adjusting plate, and adjusting the gap between the second operating plate and the adjusting plate.

CONSTITUTION: The first operating plate 5 on the diaphragm side is set horizontal to the bottom face of a main frame section 1 at the preset temperature, and the second operating plate 17 is set parallel with the horizontal first operating plate 5 when positions of a support shaft 6 and a support shaft 18 are specified. The preset temperature is not changed even if a proportional band adjusting ball 22 is moved with a proportional band adjusting dial 21. The position of the free end 27b of a wiper contact piece 27 for a potentiometer 29 can be easily adjusting by rotating an gap adjusting screw 25.