A thermally responsive switch includes a hermetic container (2) including a metal housing (3) and a header plate (4), at least one conductive terminal pin (10A, 10B) hermetically fixed in the through hole (4A, 4B), a fixed contact (8) fixed to the terminal pin (10A, 10B) in the container (2), a thermally responsive plate (6) having one end connected and fixed to an inner surface of the container (2) and reversing a direction of curvature at a predetermined temperature, and at least one movable contact (7) secured to the other end of the thermally responsive plate (6). Each of the fixed contact (8) and the movable contact (7) includes a silver-tin oxide-indium oxide system contact, and the container (2) is filled with a gas containing helium ranging from 50% to 95% so that an internal pressure of the container (2) ranges from 0.25 to 0.8 atm. at room temperature.