PURPOSE: To prevent damaging of a fixing device and to improve reliability by providing a means for shutting off a heat source by the detection signal for an abnormal temp. from a detecting circuit provided separately from a temp. control system.

CONSTITUTION: The temp. of a fixing device is judged to be abnormal if the voltages VT, VF in a measuring circuit 10 for the temp. of the fixing device and an upper limit circuit 12 for the fixing device are VT≥VF. A CPU4 makes the signal to a firing element 7 non-active in this stage and therefore the abnormal temp. rise does not arise if the element 7 is normal. The damage of the fixing device is prevented even when the element 7 is broken by a short circuit mode or when the CPU4 is abnormal. More specifically, a system for another comparator 13 is provided in addition to the control system for the CPU4 so that a relay 14 turns on and the power source for a lamp 6 is interrupted from the source when the temp. of the fixing device attains the abnormal temp. limit VF in the period after lapse of the time until overshoot settles down.