PURPOSE: To shorten a set time and to reduce consumption of electric power with a simple constitution, by covering and heat insulating a prescribed metallic plate forming the inner wall of a thermostat.

CONSTITUTION: A metallic plate 3 such as stainless steel etc. forming the inner wall of a thermostat is covered by a heat insulating material 2 such as silica base sponge etc. and a constant temperature space is not affected by the metallic plate having a large heat capacity. Further, the setting of the thermostat for low or high temperature is carried out quickly and also, consumption of electric power etc. is reduced and running cost of the thermostat is also reduced.