An image decoding method comprising: decoding information representing that a first block of an image has been encoded in a mode selected from a group of modes comprising: a first mode, a second mode and a third mode; and reconstructing the first block by setting pixel values of the first block to be identical to pixel values of a second block of the image, which is adjacent to the first block and has been decoded prior to the first block, based on the decoded information when the decoded information represents that the first block has been encoded in the first mode, wherein the first mode is a mode for encoding information representing that the first block is identical to the second block and has been encoded in the first mode, instead of encoding the pixel values of the first block, wherein: the second block has been decoded in the first mode, or in the second mode in which the second block is encoded by performing discrete cosine transformation, or in the third mode in which the second block is encoded based on a plurality of bit planes of pixel values, the image deccoding method further comprising: if the decoded information that indicates that the first block has been encoded in the second mode, decoding the first block by parsing direct current coefficients from among discrete cosine transformation coefficients included in the bitstream, restoring a plurality of bit planes of alternating current coefficients from among the DCT coefficients using a bit plane-based decoding method, and performing inverse DCT on the restored AC coefficients and the parsed DC coefficients, wherein the second mode is a mode for encoding the first block of the image by performing DCT. decoding information representing that a first block of an image has been encoded in a mode selected from a group of modes comprising: a first mode, a second mode and a third mode; and reconstructing the first block by setting pixel values of the first block to be identical to pixel values of a second block of the image, which is adjacent to the first block and has been decoded prior to the first block, based on the decoded information when the decoded information represents that the first block has been encoded in the first mode, wherein the first mode is a mode for encoding information representing that the first block is identical to the second block and has been encoded in the first mode, instead of encoding the pixel values of the first block, wherein: the second block has been decoded in the first mode, or in the second mode in which the second block is encoded by performing discrete cosine transformation, or in the third mode in which the second block is encoded based on a plurality of bit planes of pixel values, the image deccoding method further comprising: if the decoded information that indicates that the first block has been encoded in the second mode, decoding the first block by parsing direct current coefficients from among discrete cosine transformation coefficients included in the bitstream, restoring a plurality of bit planes of alternating current coefficients from among the DCT coefficients using a bit plane-based decoding method, and performing inverse DCT on the restored AC coefficients and the parsed DC coefficients, wherein the second mode is a mode for encoding the first block of the image by performing DCT. the second block has been decoded in the first mode, or in the second mode in which the second block is encoded by performing discrete cosine transformation, or in the third mode in which the second block is encoded based on a plurality of bit planes of pixel values, the image deccoding method further comprising: if the decoded information that indicates that the first block has been encoded in the second mode, decoding the first block by parsing direct current coefficients from among discrete cosine transformation coefficients included in the bitstream, restoring a plurality of bit planes of alternating current coefficients from among the DCT coefficients using a bit plane-based decoding method, and performing inverse DCT on the restored AC coefficients and the parsed DC coefficients, wherein the second mode is a mode for encoding the first block of the image by performing DCT.