PURPOSE: To prevent the heat from a heater part from exerting a bad effect on a contact opening and closing mechanism part.

CONSTITUTION: Since partition walls 21, 22 are provided with voids 14a-14c, even when heat is generated by a heater part in chambers A1-A3, the heat can be released to the outside through the voids 14a-14c to prevent the rise of the interior temperature on the heater part side, and the rise of the interior temperature on a contact opening and closing mechanism part side can be suppressed. Since the chamber A4 of the contact opening and closing mechanism part is never influenced, and the temperature of a correction bimetal 7 can be kept almost uniformly low, the range of effective temperature can be extended, the range of the regulating temperature for regulating the operation characteristic of the contact opening and closing mechanism can be extended, regulating work is improved, and assembling performance is more enhanced.