In an image forming apparatus of the present invention, an allowable temperature range of a fixing unit is divided into a plurality of fixing temperature ranges, and image data conversion .gamma. tables respectively suitable for the divided fixing temperature ranges are prepared. A fixing temperature of the fixing unit is measured during image forming. A .gamma. table suitable for the measured fixing temperature can be selected and used. Consequently, even when a reduction in fixing temperature within the allowable temperature range is caused in the fixing unit in the case of continuously outputting color images at a high speed, the resulting change in tint of output images can be reduced.