PURPOSE: To suppress variations in temperature and lamp output by inputting output signal of temperature detecting means to the PID control circuit of a controller through a phase advance compensator.

CONSTITUTION: The output of a temperature detector 2 is input to the phase advancing circuit 12 of a controller 10 through an A/D converter 4. The circuit 12 compares a measured temperature with a target temperature, and a transfer advance is compensated. Its output is input to a PID control circuit 11, PID calculation is performed based on the compensated temperature measured value to obtain a control amount of a lamp 1. The output controls the lamp 1 through a D/A converter 7 and a thyristor 8.