An image processing apparatus is configured to carry out image processing on image data including pixel values of a plurality of pixels disposed on a plane defined in a horizontal direction and a vertical direction. The apparatus includes horizontal-direction image-processing means for carrying out image processing in the horizontal direction on a pixel value of a target pixel that is included in the image data; vertical-direction image-processing means for carrying out image processing in the vertical direction on the pixel value of the target pixel; diagonal-direction image-processing means carrying out image processing in a predetermined diagonal direction on the pixel value of the target; correction-value determination means for determining the correction value of the pixel value on the basis of processing results obtained by each means; and correction means correcting the pixel value of the target pixel.