To improve the reproducibility of an image density gradation to the PWM of an image to be formed.

A single fundamental color is set for input image data consisting of fundamental colors such as yellow, magenta, cyan and black (S21) and is divided into blocks of specified size in a main scan direction (S22). In addition, an average value is sought for each of the density values of these blocks (S23), and the density values are sorted in terms of level by the magnitude of the average value (S24). When it comes to an intermediate density, the gradation is converted using a rearrangement matrix with a maximum weighted position arranged at a different position per color (S25). Further, of a low density area and a high density area, the gradation is changed using the rearrangement matrix with the changed weighted position (S26, S27). This process is repeated in the entire auxiliary scan direction (S28) for all the fundamental colors (S30).