An image encoding device and corresponding decoding device are disclosed for achieving a high rate of compression. The encoding device includes an image conversion module that converts the direction of the images using right-left symmetry mode convention or up-down symmetry mode conversion. Encoding modules are used to encode both the unconverted and converted images. A mode selector module compares the quantity of codes in the encoded unconverted images and the quantity of codes in the encoded converted images, and selects codes that are smaller in the quantity of codes for output.