PURPOSE: To execute efficient cooling considering the calorific value of component parts associated with the miniaturization of an image forming device such as a printer.

CONSTITUTION: By attaching a fan 23 to one side frame 32b and arranging a fin 32e constituting a splitting means at a position where it is proximately opposed to the fan 23, air flow generated by the fan 23 is split in two directions, up and down. The air flow split to the upper side is led to the inside of a duct 28. A fixing part having a heating roller is attached to the duct 28 and the duct 28 whose temperature rises by absorbing the generated heat from the heating roller is cooled by the air flow passing inside. As a result, a discharged toner box oppositely arranged through the duct 28 is not thermally influenced by the heating roller in substance. The air flow split to the lower side successively cools a power source unit 22, control substrates 20 and 21 and a motor 24 which are arranged in order of the calorific value from the smallest one, and is discharged to the outside from the other side frame 32a.