To almost equalize the power of (n) lines of light beams on a photoreceptor surface only by inputting only one set value of target output of a light beam to the light beam controller of an image forming device having a beams scanning system using the (n) lines of light beams.

This light beam output controller is equipped with optical detecting means 14 and 15, which detect light output of light beams, a command setting means 10, which sets the command of light output, and control means 16 and 17, which compare the output signals from the optical detecting means 14 and 15 with a reference signal Vref outputted and controls the light output intensity of the light beams according to the comparison output for the (n) lines of light beams respectively. The command setting means 10 is equipped with a setting-changeable reference signal correction value setting means 14 for each of the 2nd and succeeding light beam and uses a reference signal V'ref generated by operating the 1st reference signal with a reference signal correction value corresponding to each light beam as the reference signal for the 2nd and succeeding light beams.