Differing ratios of the amount of light from light source LD2 to the amount of light from light source LD1 are provided. An image pattern for adjustment that includes, for each color, an area made up of isolated dots by light source LD1 and an area made up of isolated dots by light source LD2, is printed for each ratio. In a copier, a density of each area is detected, a ratio at which a difference between the density of an area of isolated dots by light source LD1 and the density of an area of isolated dots by light source LD2 is smallest is decided as an adjustment ratio, and control is performed such that light sources LD1 and LD2 emit light at the adjusted ratio.