A system enables information transmission between a server which serves information by its own communication format and a client which receives service on a general network, with an efficient and simple structure. A conversion server 101 receives a video transmission request message from a client 103 in the HTTP protocol, the conversion server 101 converts the message into a message of a format of a server 102, and transmits the converted message to the server 102. Then, the conversion server 101 returns video data transferred from the server 102 to the client 103 in the HTTP protocol.