An image forming apparatus comprises an image data generating unit configured to convert a tone of an input value which indicates a density of a pixel by using a predetermined dither matrix and generate image data. The image forming apparatus further comprises a drive source and a gear configured to transmit a drive force from the drive source to an image carrier. The dither matrix includes a plurality of sub-matrixes arranged in a predetermined rule and a dot in each of the plurality of the sub-matrixes grows from a corresponding original point. The image forming apparatus satisfies a relation of (1) a≧0.24 mm and b/a 1.2, where “a” is a travel distance of a printing medium per tooth of the gear in a secondary scanning direction orthogonal to the primary scanning direction, and “b” is a component in the secondary scanning direction of a distance between the original point of the dot derived from a first sub-matrix and the original point of the dot derived from a second sub-matrix.