An image forming apparatus switches a line of image data, to be read out from a storage unit, to another, depending on a position in the line in a scanning direction such that the curving of a scanning line is offset, and reducing trouble in reading out image data in units of rectangular image data blocks A data processing section performs image processing based on a plurality of rectangular image areas generated by dividing image data stored in a memory section. An image reading unit reads out the image data as the rectangular image areas, and transfers the rectangular image areas to the data processing section. A printer section forms an image by scanning a photosensitive member with irradiation light based on the rectangular image data areas. A DMA controller stores positional information indicative of line-switching positions. The rectangular image data areas are read out according to the positional information.