PURPOSE: To eliminate unstable contact or arc discharge and extend the life of the contact point of a heat responsive switch by connecting a resistance in series to a heater and connecting a switching element in parallel to the resistance.

CONSTITUTION: When external light is dark and the contact point 3 of a heat responsive switch SW is closed, the voltage at both ends of a resistance R3 is low and a switching element remains turned off. When the illuminance increases gradually, the resistance of a photoelectric element 2 decreases, the current flowing through a heater H increases, and an element 5 is turned on to short-circuit the resistance R3. As a result, the amount of the heat generated by the heater H increases suddenly to open the contact point 3 and to turn a lighting load 4 off. When the external light changes from a bright condition gradually to a dark condition, the resistance of the element 2 increases gradually, the current flowing through the heater H decreases, and the element 5 is turned off to interpose the series resistance R3. This lowers the current flow through the heater H suddenly and causes the contact point 3 to close and to turn the load 4 on. As a result, unstable contact and arc discharge of the switch SW are eliminated and the life of the switch is extended.