An image forming apparatus forms an image by scanning on a photosensitive member in the main-scanning direction with a plurality of beams aligned in the sub-scanning direction. To detect differences in the main-scanning length of each beam on a photosensitive member, patterns for correction are formed on the photosensitive member using first and second beams at both ends among the plurality of beams. The positions of a first pixel and a last pixel in the main scanning direction on the photosensitive member, which pixels are to be formed by a third beam arranged between the first and second beams, are determined based on the detected first and second locations of the first and second test patterns respectively formed by the first and second beams, without forming a third test pattern by the third beam.