Abstract: Before a start frame from which decoding of a bit stream is to be started and a reference image frame which is decoded in order to obtain a reference image necessary to decode the start frame are decoded, a quantization matrix stored in a quantization matrix buffer is updated such that, where a quantization matrix is described in any of B pictures disposed between the start frame to be decoded or the reference image frame and an I picture or a P picture preceding in time to the start frame or the reference image frame, the start frame and the reference image frame are decoded with the quantization matrix described in the B picture. Then, where a quantization matrix is described in the frame to be decoded, a decoding process of the bit stream is performed using the quantization matrix and the quantization matrix buffer is updated using the quantization matrix.