To solve the problem that, in irrational number screen processing, a screen shape is determined freely on the basis of an angle of a screen and the number of lines but, meanwhile, the scale of hardware for coordinate transform arithmetic operation or the like using a triangular function is expanded.

An image processing apparatus performs screen processing of image data constituted of density data in a plurality of colors using: a rational number screen processing constituted of a number less the colors constituting the image data; and an irrational number screen processing means constituted of a number less than the colors constituting the image data. By switching screen processing on a color component unit basis, optimal screen processing is performed without expanding the circuit scale of hardware.