An image forming apparatus includes a plurality of photosensitive drums respectively corresponding to a plurality of colors to print, on a recording medium, color chart of a plurality of vertical lines in a sub-scanning direction to a main scanning direction of the recording medium and to be arranged at intervals along the main scanning direction. A vertical line of the plurality of vertical lines includes at least two colored sub-lines to be formed by corresponding at least two photosensitive drums. A processor is to perform a correction in non-linear color registrations in the main scanning direction using an image of the color chart.