A data stream of a code string is divided into a series of a group of blocks. The first block of the group of blocks is compressed by a sliding dictionary method with codes of the first block as a reference dictionary. Blocks subsequent to the first block are recognized in succession as a target block to be compressed. Each time the target block is recognized, the target block is compressed by the sliding dictionary method with codes of the target block and results of compression of a group of blocks prior to the target block as a reference dictionary, and a result of compression of the target block is arranged at an end of the results of compression of the group of prior blocks.