An image forming apparatus for easily detecting occurrence of positional deviation and magnification deviation among images, even when the deviations are small. Reference patches, which are rectangular areas, in which a longitudinal direction is a sub-scanning direction, painted in magenta, are formed on paper using a first image forming device. A target patch constituted by rectangular unit patches arranged in the sub-scanning direction is formed on the left side of the reference patch. Another target patch constituted by rectangular unit patches arranged in the sub-scanning direction is formed on the right side of the reference patch. The more downward each of the unit patches are positioned at, the more to the right the unit patches are shifted.