Multicast file transmission method

A multicast file transmission method is disclosed, which permits efficient multicast file transmission with high reliability even in the communication system, which uses in the up-link direction a specified transmission circuit, such as a multiple access satellite communication circuit. In the multicast file transmission method, a data file is composed of a plurality of blocks from a sending side to respective receiving destinations. At respective destinations, a test is carried out to find whether or not any transmitted packet is erroneous in the data file composed of a plurality of blocks after the end of said transmission of the data file. To the sending side from any of the respective receiving destinations, where any transmitted packet is found erroneous by the testing in the data file composed of a plurality of blocks after the end of said transmission of the data file, a request-for-retransmission signal is transmitted for requesting retransmission of the transmitted packet of data found erroneous in the data file received at any of respective receiving destinations. The request-for-retransmission signal is defined to indicate the offset position of the erroneous data in the transmitted packet of data and the data length of the erroneous data.