An optical scanning device which forms a plurality of scanning lines in parallel in a main scanning direction on a photosensitive member by a plurality of laser beams detects the curvature values in the sub-scanning direction of the scanning lines, and corrects a curvature value in the sub-scanning direction corresponding to each of the scanning lines. When detecting the curvature value, the optical scanning device detects the curvature values in the sub-scanning direction of two scanning lines, and calculates the curvature value in the sub-scanning direction of a scanning line between the two scanning lines based on the detected curvature values in the sub-scanning direction of the two scanning lines. With this arrangement, the curvatures in the sub-scanning direction of scanning lines are corrected effectively with high accuracy for all laser beams in a multi-beam optical scanning device using a VCSEL or the like.