High quality images are provided without the occurrence of image quality defects due to reciprocity law failures, even when scanning a multi-beam. When interlaced scanning is employed, it is possible to select combinations such that, taking any freely selected neighboring scanning lines, respective scan numbers of the neighboring scanning lines are always different. Thus, a duration of scanning spacing can be made to be at least the duration of a single main scan. As a result, image defects such as banding due to reciprocity law failures can be greatly alleviated, and images can be provided in which image defects are to unlikely to be noticed in practice.