If an edge is detected between each pair of two mutually adjacent pixels among three serially adjacent pixels in a digital image, a candidate pixel selection unit in an intermediate value pixel judgment unit selects a pixel located at the middle among the three pixels as a candidate pixel. A pixel value comparison unit compares the pixel values of the three pixels, which are used for selecting the candidate pixel, with each other. Only if the pixel values of the three pixels monotonically increase or monotonically decrease along the arrangement direction of the pixels, a judgment unit judges that the candidate pixel is an intermediate value pixel.