To suppress voltage fluctuation caused by turned ON/OFF of a lamp, by applying normal lamp actuating voltage, after preliminarily heating the lamp, while gradually applying from the lower voltage to the higher voltage, to either of the fixing lamp and the exposing lamp.

The exposing lamp 5 is preliminarily heated for a specified time, while applied with output voltage by a step-down transformer 10 in order from the lower one, before entering the exposing operation, and thereafter it is switched so that the normal application voltage V0 is applied. Then, as to the fixing lamp 4, at the first time turned on, the output voltage of the step-down transformer 10 is also applied in the order from the lower one. Thereafter, the output voltage V1 of the step-down transformer 10 is applied at the time of standing by, the voltage V1 is turned on and off corresponding to the output of a temp. detector 13, and a fixing device is kept hold at the preliminary heating temp. being lower than the fixing operation temp. Moreover, at the time of switching the voltage applied to the fixing lamp 4 and the exposing lamp 5, the switching relays 8 and 9 are switched by temporarily turning off the respective switching element 6 and 7, and thereafter the fixing lamp switching element 6 or the exposing lamp switching element 7 is turned on.