In an image forming apparatus, a first sequence group operates after input of a print signal and a second sequence group is started up at timing when a temperature T of a fixing device has reached a certain temperature T1 after the input of the print signal or timing for which a temperature rising curve is calculated from a time for the temperature T to reach a certain temperature T2 from the temperature T1 during temperature rising of the fixing device. With this image forming apparatus, it becomes possible to reduce unnecessary operations of a developing device and a photosensitive drum and extend a usable life of the developing device even if image formation and temperature rising of the fixing device are performed in parallel.