To relieve a communication load on multi-cast transmission processing to transmit multimedia data in a video-on-demand system or the like to pluralities of clients.

In transmission processing of multi-cast data in an ATM-LAN environment consisting of pluralities of clients 41 that receives multicast data, a multicast server 11 and a multicast data transmission request client 12, the transmission request client 12 requesting transmission of the multicast data to the multicast server 11 is in existence in the same computer as the multicast server. In this case, in the case of transmitting the multicast data requested for the transmission to the multicast server 11, the transmission data is transferred in the computer where the multicast server 11 is in existence by a function call not via the ATM-LAN.