A compression engine starts compressing data by a preset first compression rule, compresses the following data by a second compression rule when the characteristics of the data satisfy a predetermined switching condition, and returns to the first compression rule when the characteristics of the data do not satisfy the switching condition to compress the data and the following data. A decompression engine starts decompressing compressed data by a first decompression rule corresponding to the first compression rule, decompresses the following compressed data by a second decompression rule corresponding to the second compression rule when the characteristics of the data after decompression satisfy the switching condition, and returns to the first decompression rule when the characteristics of the data after decompression do not satisfy the switching condition to decompress the data and the following compressed data.