PURPOSE: To allow an incandescent lamp to retain brightness even when it is out by connecting in parallel a photocoupler to which a zero cross circuit is connected to a rectifier diode, and using one half wave of an ac power source to cause a triode AC switch to be constantly ignited by means of a diode while using the other half wave to intermittently turn the coupler on and off.

CONSTITUTION: When ac power source is turned on, a rectifier diode D is used to cause an immediate half wave ignition of a triode AC switch 2 to light up an incandescent lamp 1. Ac power is applied to an oscillation circuit 5 while dc power is supplied to generate square waves of a set period, which are sent via a driver 4 to activate a photocoupler composed of luminating and luminated elements. Since the coupler 3 is adapted to ignite a triode AC switch 2 around the zero cross point of the AC power source, the triode AC switch 2 enters full wave ignition state to supply the full waves of the AC power source to the incandescent lamp 1 while the coupler is operating and the triode AC switch 2 enters half wave ignition state to supply the half waves of tone AC power source to the incandescent lamp 1 while the coupler is not operating.