The present invention provides a high speed MMR compression processing method by reducing the number of times of repeating a process of finding the positions of changing pixels a1, a2, b1, and b2. According to this method, first, a bit string corresponding to pixel data is obtained from an image. Then, data are sequentially fetched from the bit string byte by byte. Subsequently, a search is made of a table that describes the number of consecutive bits that are arranged to the right of and have the same color as each of bits of each byte of all bit patterns represented by bits included in the fetched 1 byte. Then, the number of consecutive bits, which are arranged to the right of and have the same color as each of bits of each of the fetched bytes, is detected according to the bit patterns represented by bits of the fetched byte. Subsequently, a pixel placed at the position corresponding to the number obtained by adding 1 to the detected number of consecutive bits is determined as a color changing pixel. Then, the position of each changing pixel in each of the sequentially fetched bytes is obtained. Subsequently, the image data is coded according to the position of each of the changing pixels.