A communication system includes a plurality of terminals and a server that relays packet data transmitted and received between a first terminal and a second terminal among the plurality of terminals. The server includes a connection establishment message processing unit configured to transmit a first SYN ACK message that is a response message to a first SYN message transmitted from the first terminal for establishing a TCP connection upon receiving the first SYN message from the first terminal, and an option information notifying unit configured to transmit a first TCP option information notifying message that includes first TCP option information included in the first SYN message from the first terminal and first communication destination information to indicate the second terminal as a destination of communication. The first terminal recognizes the second terminal as the destination of communication and notifies the second terminal of the first TCP option information included in the first TCP option information notifying message, upon receiving the first TCP option information notifying message from the option information notifying unit.