It is determined whether the pixel of interest in image data to be processed belongs to area 1 not close to a scan line changing point where a scan line changing process is done (S101). If the pixel of interest belongs to area 1, an error diffusion process is performed using an error diffusion matrix for area 1 (S102). If the pixel of interest belongs to area 2 close to the scan line changing point, the error diffusion process is performing using an error diffusion matrix for downward scan line changing for area 2 when the scan line changing process is changing to a lower line, or an error diffusion matrix for upward scan line changing for area 2 when the scan line changing process is changing to an upper line.