An image forming method is performed by an image forming apparatus having a head mounting a plurality of writing units that form an image on a recording medium. The writing units are aligned in a sub-scanning direction perpendicular to a main-scanning direction. A part of the image is formed on the recording medium by the writing units, the part of the image forming a line extending in the main-scanning direction. The step of forming a part of the image is repeated while shifting the writing units in the sub-scanning direction by a predetermined amount of shift. The amount of shift of the wiring units is varied so that an amount of misalignment in a connecting portion between the parts of the image on the recording medium is reduced.