A multicolor image forming apparatus includes an exposure unit to direct optical beams for optically writing different single-color images on image carriers, respectively, a pattern forming unit to form a positioning pattern on a transport member, a pattern detector to detect the positioning pattern, disposed above the transport member, a positional data detector disposed on a scanning line to detect positional data in a sub-scanning direction of the optical beams, an adjustment unit, and a storage unit. The adjustment unit detects positional deviations among the different single-color images based on detection results generated by both the pattern detector and the positional data detector, respectively, and then corrects the positional deviations. The storage unit stores as reference data the positional data in the sub-scanning direction of the optical beams detected when the positional deviations are corrected.