PURPOSE: To eliminate overshoot when heating with a heater and to execute a stable temperature control, by applying high voltage to a heater provided on a lamp at the time of a reference temperature or below, and switching it to low voltage at the time of reference temperature or above.

CONSTITUTION: Immediately after an electric power source is turned on, or under low temperature environment, high voltage V1 is supplied to a heater 2 through a relay contact PL1, therefore, a lamp tube wall temperature rises quickly. Subseqently, when the tube wall temperature reaches a prescribed value, a thermostat contact THS2 is closed and a relay coil RLC is energized. As a result, a relay contact PL2 is closed, and low voltage V2 is applied to the heater 2, therefore, a rise of the tube wall temperature becomes gentle and overshoot becomes small. Thereafter, the heater 2 is subjected to a temperature control by opening and closing of a thermostat contact THS1. In this way, overshoot in case of heating by a heater is eliminated and a stable temperature control can be executed.