A communication system includes: a relay device including: a storage unit configured to store terminal specification information for specifying the third terminal associated with the second terminal, and a first communication control unit configured to connect the third terminal to the first network if an authentication control unit authenticates the terminal authentication information; one or more first terminals connected to a first network; a second terminal connectable to the first network and including a second communication control unit configured to transmit the relay-device specification information and the terminal authentication information to the third terminal through the second network if receiving the relay-device specification information and the terminal authentication information from the relay device; and a third terminal connectable to a second network and including a third communication control unit configured to transmit the terminal authentication information to the relay device through the second network.