The optical scanning apparatus correctly measures a gap between the laser beams of a plurality of the laser beams even when the scanning speed is changed and when the variation is caused, rightly corrects the variation. Thus, the optical scanning apparatus inputs detection signal BD_IN to the edge detecting unit at timing at which the laser beam is detected. During the enable gating, the count value representing the distance between the head laser beam and the end laser beam is counted in the counting unit. A plurality of speed ratio parameters according to the degree of changes in the scanning speed is input in the selector. The selector selects any one of the speed ratio parameters and multiplies by the count value by the multiplier, it is controlled to obtain the same multiplication result whichever scanning speed may be selected.