Image forming apparatus

An image forming apparatus includes a photosensitive drum which is rotatable; light-emitting portions arranged along a direction of a rotational axis of the photosensitive drum, the light-emitting portions emitting light based on image data in order to expose the photosensitive drum; a reference clock signal generation portion which generates a reference clock signal with a predetermined period; a modulated clock signal generation portion which generates a modulated clock signal with a predetermined modulation period by spreading a spectrum of the reference clock signal and by modulating the predetermined period, wherein the light-emitting portions emit light for a lighting time set based on the modulated clock signal; and a control signal generation portion which generates with a period of n times the predetermined modulation period (n is an integer greater than or equal to 1) a control signal for controlling a timing at which the light-emitting portions emit light.