PURPOSE: To perform the picture forming preparatory processing effectively in a fixed time, by detecting that the heating fixing means has reached a prescribed detection temperature during the warming-up time.

CONSTITUTION: Thermoelectric element 18 such as a thermistor is brought into contact with the circumferential face of roller 12 to detect the circumferential face temperature of roller 12. The signal of thermoelectric element 18 is applied to the first W the foruth comparing means 19W22 to compare the detection temperature of element 18 with reference temperatures set in means 19W22. Signals of means 19W22 are applied to control circuit 24 such as a microcomputer, and control circuit 24 receives signals of respective means to control heater 12', the rotation driving motor of drum 1, lamps 4 and 5, and corona dischargers 2 and 3 on a basis of the program programmed previously. As a result, the picture forming preparatory processing is performed efficiently and stably even for change of the external environment.