A data compression program making a computer perform processing which enables compression of only highly compressible data. First, a transmitter-side device performs compression processing of a portion of data to be compressed. Next, the transmitter-side device compares volumes of the portion of the data before and after the compression processing, and determines whether or not the compression processing is effective. When the data to be compressed is uncompressed data, the volume of the portion of the data is reduced by the compression processing. On the other hand, when the data to be compressed is compressed data, the volume of the portion of the data is not reduced by the compression processing. Therefore, only when it is determined that the compression processing is effective, the transmitter-side device compresses the other portions of the data to be compressed.