An image forming apparatus includes plural toner adhered amount detectors that detect an amount of toner particles adhered to the toner image on the image bearer and an image density controller to form a gradation pattern by sequentially arranging plural toner patches in the sub-scanning direction on the image bearer from the least toner adhered amount patch. These toner patches bear a different amount of toner particles from each other. The plural toner patches are detected by the plural toner adhered amount detectors, and the image density controller adjusts an image formation condition for the image formation device based on detection values thereof. These toner adhered amount detectors are symmetrically aligned in a main scanning direction above the image bearer. The image density controller forms the plural toner patches being staggered at different positions on the image bearer, facing these toner adhered amount detectors, respectively.