In a multibeam-type image printing apparatus, a process of adjusting a beam scanning position at which a photosensitive body is scanned is simplified. More specifically, consumption of a developing agent such as toner or the like is suppressed, and the time required for the adjustment process is shortened. A test pattern is formed on the photosensitive body using the first beam out of a plurality of beams, thereby detecting the irradiation position of the first beam with respect to the photosensitive body. Correction data for all the beams are generated on the basis of adjustment data as the detection result and predetermined beam information (information indicating a relative positional relationship between the plurality of beams on the photosensitive body). Image transfer clocks and scanning start positions are set on the basis of the correction data.