An image forming system is provided that uses printing apparatus groups each consisting of a plurality of printing apparatuses. The printing apparatuses hold print heads each provided with nozzles arrayed in a predetermined direction, are arranged two-dimensionally to cooperate with each other in printing rasters extending in the predetermined direction. Two printing apparatus groups are arranged in a medium conveying direction. In forming images in areas divided in the predetermined direction, the printing apparatuses or print heads located on the upstream side and the downstream side are appropriately set to participate in the printing of the same raster. In printing for one divided area, when only one print head is used to reduce electric power consumption or when two print heads are used for faster printing or for higher print quality, a selection is made of a combination of printing apparatuses or print heads that considers the printed image quality.