To detect the misalignments and densities of toner images by a simple structure.

In order to correct the misalignments of toner images formed by image forming parts, at least three or more position-detection toner marks B are formed in a main scanning direction perpendicular to the carrying direction of a carrying belt. The misalignments of the position-detection toner marks B are detected by outputs from sensors 17 to 19. The density of a density- detection toner patch A formed on the carrying belt in order to correct the density of the toner image formed by each of the image forming parts is detected by at least one of the sensors 17 to 19 used for detecting the position- detection toner marks B. Thus, the misalignments of the toner images and the densities of them can be detected by the simple structures.