To reduce unevenness of density of an image in the main scanning direction of a light beam.

An up/down signal SCUD is H in section A1 and down count is performed at the rising timing of a clock signal SCCLK. A driving voltage Vdrv decreases as the count decrements thus reducing the intensity of a light beam being emitted from an LD gradually. Since the up/down signal SCUD is L in section A2, up count is performed and the driving voltage Vdrv increases as the count increments thus increasing the intensity of a light beam gradually. As the frequency of the clock signal SCCLK is higher as compared with that in section A1, variation rate of the driving voltage Vdrv is increased. Intensity of a light beam in the image area is corrected in a desired pattern such that unevenness of density of an image is corrected in the main scanning direction of a light beam by switching the up/down signal SCUD between H and L thereby varying the frequency of the clock signal SCCLK.