To provide a thermostat having a reduced width size to be easy and inexpensive to mount on a seat heater, etc.

The thermostat 1 includes a temperature control bimetal plate 6 shaped approximately circular, and a temperature overrise preventing bimetal plate 11 shaped rectangular with its size in the direction of neighboring the temperature control bimetal plate 6 being smaller than its size in the direction of a terminal 12 perpendicular to the direction of neighboring the temperature control bimetal plate 6, the approximately circular temperature control bimetal plate 6 having a notch 6a in part of the circular shape so that its size in the direction of neighboring the temperature overrise preventing bimetal plate 11 is smaller. Thus, the thermostat 1 has a reduced width size to be easy and inexpensive to mount on the seat heater.