In a fixing device of an image forming apparatus, at least one roller of a pair of rollers (14, 15) that come into pressure contact with each other is regulated so that outer diameter of the roller is 80 to 120 mm and total length of the roller (length in the axial direction) is 470 to 600 mm, and the roller is driven and rotated at the number of revolutions of 60 to 300 rpm by fitting a key groove (14c) formed at an end of the roller to a drive key (27a) provided for a drive force-transmitting shaft. Further, average thickness of core metal material (14a) of the fixing roller is regulated so that the moment of inertia in a direction of a rotational shaft of the fixing roller is 70 Kg·cm2 and less.