Directional data is generated representing a pattern that indicates a growing direction of a dot on a scanning line. Coefficients used to distribute the image data of a pixel of interest are set. An edge of an input image is detected. The image data of the pixel of interest of the input image is distributed to the interest pixel and a pixel adjacent to the interest pixel based on the coefficients. Image data distributed from the adjacent pixel is added to image data of the interest pixel on which the distribution is performed, and the sum of the image data is as corrected image data of the interest pixel. The corrected image data and the generated directional data are selected for an edge, and the image data of the interest pixel and input directional data are selected for a non-edge to generate an image signal pulse-width modulated.