An immediate printing type image forming apparatus is provided of which the warm-up time is sufficiently shortened without causing any trouble to occur in the execution of image reading processing and/or image forming processing. The image forming apparatus is configured to determine a time required to raise the current temperature of the surface of a heating roller in a fixing unit to a predetermined temperature and increase the duty ratio of a power waveform to be fed to each of a copy lamp and a motor driving a polygon mirror when the time required thus determined is shorter than each of a time required to increase the current light quantity of the copy lamp to a predetermined light quantity and a time required to raise the current rotational speed of the motor to a predetermined speed, thereby turning each of the copy lamp and the motor into respective predetermined condition before the elapse of the determined time required.