A thermally responsive switch includes a container including a metal housing and a header plate, at least one conductive terminal pin inserted through a hole of the header plate, a fixed contact fixed to the terminal pin, a thermally responsive plate having one of two ends conductively connected via a support to an inner surface of the container and reversing a direction of curvature at a predetermined temperature, and at least one movable contact electrically conductively secured directly to the other end of the thermally responsive plate. Each contact is comprised of a silver-cadmium oxide system contact. The container is filled with a gas containing helium ranging from 50% to 95% so that an internal pressure of the container ranges from 0.38 atmosphere to 0.68 atmosphere at room temperature such that arc generated during opening of the contacts moves on surfaces of the contacts without spreading from the contacts.