To avoid image unevenness caused by light quantity unevenness between light sources by adjusting the light quantity of a light beam outputted from each light source with high accuracy in an arbitrary time, without inflating cost.

Photosensors PS1 and PS2 are inserted between exposure heads CH4 and CH7 and a drum 14 by a moving mechanism, light quantity of each of laser beams L4 and L7 is detected, next, the photosensors PS2 and PS3 are inserted between the exposure heads CH4 and CH7 and the drum 14 and the light quantity of each of the laser beams L4 and L7 is detected. After calibrating sensitivity of the photosensors PS1-PS3 from these detected light quantities, the photosensors PS1-PS3 are inserted between the exposure heads CH1-CH9 and the drum 14, and light quantity adjustment of the laser beams L1-L9 is performed.