The image resolution of an original image is reduced, so that the original image is changed to a low-resolution image. Here, a bi-linear interpolation or a bi-cubic convolution is carried out for the resolution reduction. Pixel data from each pixel of the low-resolution image is filtered by a low-pass filter. Due to this, the low-resolution image changes to a filtered image. The number of pixels forming the filtered image is restored to the number of pixels forming the original image to generate a restored image. In the restored image, the contours of the objects are sufficiently unclear even if a small matrix is used in the filtration.