Provided is a method for transmitting a file based on a multiplex forwarder using a session join time interval. The method includes the steps of: a) transmitting a session join request message to a source and joining to a corresponding session; b) when a member joins the session, transmitting a forwarder candidate list request message to the source and receiving a forwarder candidate list; c) skipping a specific member among members on the transmitted forwarder candidate list; and d) transmitting a forwarder request message to the members on the forwarder candidate list excluding the specific member, selecting a member permitting the request as a forwarder, and receiving a gift, which is a packet, from the selected forwarder.