PURPOSE: To heat a nip roller up to a predetermined temperature in a short time by using the nip roller of poor heat respondency and a heating roller of good heat respondency and heating the nip roller by means of the heating roller in the warming-up time.

CONSTITUTION: A heating roller 1 of good heat conduction dispolsed with a heater 5 and a temperature detector 6, and a nip roller 2 of poor heat respondency are used. At the standing-by, the roller 2 is lightly contacted with the roller 1 in the same manner as that when a main switch was turned OFF, thence the rollers are rotated. When the roller 1 reaches the temperature at which fixing may be made upon final period of the warming-up, the revolution of the rollers is once stopped and the rollers 1, 2 are released of pressure or are spaced. When the copying ends, the roller 2 detaches from 1 and the revolutions of both rollers stop.