An image decoding apparatus for decoding at least a first coded bit stream into which header information of the H.263 coding scheme and image coded data encoded in the H.263 coding scheme are multiplexed, or for decoding a second coded bit stream into which header information of the MPEG-4 coding scheme and image coded data encoded in the MPEG-4 coding scheme are multiplexed, comprises a switch (102) for switching a decoding analysis scheme of decoding a direct current component of an intra-coded DCT coefficient according to the received first coded bit stream and the received second coded bit stream, wherein when the first coded bit stream is received, the switch decodes the direct current component of the intra-coded DCT coefficient by a predetermined fixed code length.