PURPOSE: To temporarily interrupt the on/off control of a heating means, to evade the fluctuation of supplied voltage and to obtain a sharp picture free of exposure unevenness by detecting the turning-on/off of an exposing lamp and maintaining the heating means in an energization or non-energization condition when the exposing lamp lights up.

CONSTITUTION: When the exposing lamp lights up, a temperature control releasing circuit 19c detects whether the halogen lamp of a heat fixing device 15 lights up or not. When the halogen lamp lights up, the temperature control releasing circuit 19c releases the temperature control by a temperature control circuit 19b to maintain the halogen lamp in a lighting condition. Conversely, when the halogen lamp goes off, the temperature control release circuit 19c releases the temperature control by the temperature control circuit 19b to maintain the halogen lamp in a go-off condition. Consequently, while the exposing lamp lights up, the temperature control of the heat fixing device 15 is temporarily interrupted.