A thermally responsive switch includes an airtight container including a metal housing and a header plate, conductive terminal pins airtightly fixed to the header plate, a fixed contact fixed to the conductive terminal pin, a thermally responsive plate one end of which is conductively connected to and fixed to the inner surface of the airtight container and the bending direction of which is reversed at a predetermined temperature, and a movable contact fixed to the other end of the thermally responsive plate. In the thermally responsive switch, the movable contact and the fixed contact are composed of a silver tin oxide based contact and gas containing 50% or more and 95% or less of helium is encapsulated in the airtight container in such a manner that gas pressure is equal to or more than 0.35 and equal to or less than 0.7 at ordinary temperature.