PURPOSE: To inexpensively change a detection set temperature by providing magnetic substance which is placed in the magnetic field of magnets and changes an operational temperature of a reed switch in the way that the strength of the magnetic field in which a contact is arranged is varied by changing the distance from the magnets.

CONSTITUTION: A prescribed temperature is detected by the reed switch 31. In the reed switch 31, the strength of the magnetic field which acts upon the contact is controlled by a ferromagnetic body 40 whose saturated flux density changes owing to a temperature, and the contact is brought into contact with a magnetic field at a prescribed temperature to detect the temperature. When the temperature to be detected is changed by changing the strength of the magnetic field which acts upon the contact while changing the distance between the ferromagnetic body 40 arranged in the magnetic field of the magnets 38 and 39 and the magnets 38 and 29 themselves, the detection set temp. is changed. Thus, a detection set temperature is inexpensively changed.