PURPOSE: To prevent a relay from chattering by setting a circuit constant so that a semiconductor switch element is turned on when the primary current of a current transformer is smaller than the starting lamp current and the under-stabilization lamp current with full lighting and greater than the preheating current flowing in a starter device using a bi-metal switch built-in a discharge lamp.

CONSTITUTION: A circuit constant is so set that a semiconductor switch element SW is turned on when the current flowing in the primary winding n1 of a current transformer TR is smaller than the starting lamp current and the under- stabilization lamp current with full lighting and greater than the preheating current flowing in a starter device which uses a bi-metal built-in a discharge lamp. Accordingly the semiconductor switch element SW remains off with the preheating current for the starter flowing when the AC power supply is turned on, and at the time of dimmer operation, No.1 switch element is turned off to shut the current flowing to the primary side of the current transformer TR, resulting in no secondary output therefrom, and the relay can be put in non-energized state. This process prevents the relay from chattering when the power supply is turned on or during dimmer operation.