An image forming apparatus includes an image forming device for forming an image on a sheet; a fixing device for fixing an image formed on a sheet; a detector for detecting a temperature of the fixing device, and a controller for controlling an image formation start timing in accordance with an output of the detector. In an operation of continuously forming images on a plurality of predetermined sheets, when a number of image formations is not less than a predetermined number, the start of the image formation is delayed until a temperature of the fixing device rises up to a first temperature, and when the number of image formations is less than the predetermined number, the image formation is started when the temperature of the fixing device rises up to a second temperature which is lower than the first temperature.