An image forming apparatus comprising: a function control processing unit which controls operations of constituent elements provided in the image forming apparatus and executes processing related to a function of the image forming apparatus; a power supply control unit for supplying power to the constituent elements; and a power supply control processing unit which is provided in the power supply control unit, starts up when power supply to the image forming apparatus is turned on, controls energization of the constituent elements, and controls an operation of the function control processing unit at a time of startup, wherein the image forming apparatus has a warming-up operation mode in which when the power supply to the image forming apparatus is turned on, the power supply control processing unit starts up, characterized in that the image forming apparatus further comprises a clock oscillation unit which transmits a clock signal to the constituent elements including the function control processing unit; and the clock oscillation unit is adapted, when power supply control processing unit starts up, to start operating to transmit the clock signal to the function control processing unit, the function control processing unit being adapted to increase its temperature by continuously receiving the clock signal, and after a lapse of a predetermined standby time set beforehand, with the function control processing unit kept inactive, the power supply control processing unit is adapted to restart itself and to start up the function control processing unit. a function control processing unit which controls operations of constituent elements provided in the image forming apparatus and executes processing related to a function of the image forming apparatus; a power supply control unit for supplying power to the constituent elements; and a power supply control processing unit which is provided in the power supply control unit, starts up when power supply to the image forming apparatus is turned on, controls energization of the constituent elements, and controls an operation of the function control processing unit at a time of startup, is provided in the power supply control unit, starts up when power supply to the image forming apparatus is turned on, controls energization of the constituent elements, and controls an operation of the function control processing unit at a time of startup, wherein the image forming apparatus has a warming-up operation mode in which when the power supply to the image forming apparatus is turned on, the power supply control processing unit starts up, characterized in that the image forming apparatus further comprises a clock oscillation unit which transmits a clock signal to the constituent elements including the function control processing unit; and the clock oscillation unit is adapted, when power supply control processing unit starts up, to start operating to transmit the clock signal to the function control processing unit, the function control processing unit being adapted to increase its temperature by continuously receiving the clock signal, and after a lapse of a predetermined standby time set beforehand, with the function control processing unit kept inactive, the power supply control processing unit is adapted to restart itself and to start up the function control processing unit.