PURPOSE: To reduce the power capacity by a method wherein an image forming device is controlled so as to reduce the power consumption when the image forming device is brought in operation while a controller is in action.

CONSTITUTION: When a copy commence instruction is issued, a manuscript on a manuscript stand a is scanned by a light exposure device 7, and the obtained image information is transferred on a sheet of paper, which is sent from a paper feed tray 4 or a paper feed cassette 5, at an image forming device 8. The paper on which the image is transferred is discharged on to a paper delivery tray 6 after undergoing fixing treatment at a fixing device 17. The set temperature of a heater 17b is kept at a normal value during copy process. On the other hand, while the book-binding device is in heating step, the set temperature of the heater 17b is lowered and also the rotating speed of a fixing roller 17a is lowered. As the transfer speed of paper at the fixing device is reduced, image forming process at the image forming device 8 is delayed correspondingly. Thereby, the power consumption during image forming process is reduced.