To properly express a multi-gradation image obtained by a multi-value dither method with the PWM(pulse width modulation system) without deteriorating the image quality by selecting a dot generating method depending on a position of a pixel in a dither matrix.

Three figures to the left side of each of elements 1-18 in a dither matrix indicates the priority of dot generation in the matrix. The growing attribute L which is the growing direction of a pixel in each element indicates growing from the left end (left growing), the growing attribute C indicates growing from the center (center growing), and the growing attribute R indicates growing from the right (right growing). When a density of a noted pixel is zero, all elements are at a 0 level. As the density of a noted pixel increases, the element 1 changes with a prescribed density width as level 1, level 2, and level 3 in the center growing, then the element 2 changes as level 1-level 3 in the right growing. Furthermore, the element 3 grows in the left growing and the element 4 grows in the center growing.