1. An image encoding apparatus comprising: a compression unit that compresses a reference image by reducing a resolution of the reference image in a resolution adjustment mode determined from among at least two resolution adjustment modes according to a distribution of values of pixels of the reference image, and that provides the compressed reference image to a memory; a reconstruction unit that reconstructs the reference image by increasing a resolution of the compressed reference image stored in the memory to an original resolution; a predictive encoding unit that performs predictive encoding on a current image by using the reconstructed reference image; and a predictive decoding unit that generates the reference image by performing decoding on the predictive encoded current image, and providing the generated reference image to the compression unit.