PURPOSE: To prevent the drop of temperature for a fixing roller even in a consecutive copying action and accordingly to ensure an accurate fixing process, by controlling the electric power of supply via a timer and by the processing which is in accordance with the external factors such as the copy mode, the fluctuation of the power supply, the environment, etc.

CONSTITUTION: The power supply peak value, the AC frequency, etc. are detected by a CPU2 incorporating an A/D converter and in accordance with the waveform supplied via a transformer 2 and a rectifier 3. The inputs given from paper size switches 11W13 and a paper quality switch 14 plus the input supplied from an atmosphere temperature detecting temperature sensor 8 via an A/D converter 9 are applied to the CPU2 as well as to a copy control CPU1 which performs an interruption processing in real time. By the processing of the CPUs 1 and 2, the timer time is controlled according to the external factors such as the copy mode, the fluctuation of the power supply, the environment, etc. Then the control is carried out to the angle of conduction with each AC middle wave which is applied to a heater 5 via a trigger circuit and a triac. With this control carried out according to the real time and requiring no detection of the temperature of a fixing roller, the drop of temperature can be avoided for the fixing roller even in a consecutive copying action.