PURPOSE: To supply electric force to a lamp stably and prolong the life of the lamp through software starting mechanism by controlling the phase of the power source voltage according to the transient state of a fixing heater which exerts influence upon variation in power source voltage.

CONSTITUTION: The controller of a lamp consists of a power source 1, halogen lamp 2, gate control rectifying circuit 5 for a triac 3, trigger element 6, zero- cross circuit A, detecting circuit 5 for the lighting operation and effective value of the lamp 2, microcomputer 27, etc. The turn-on voltage effective value of the means 5 which detects the effective value of the lamp 2 is A/D-converted by an A/D converter 29 into a turn-on voltage monitor signal, whose level is compared with the set level of the lamp 2 to perform the phase control over the lamp 2 by the microcomputer 27 according to the comparison value.