[Problem] To enable group communication with a high degree of freedom in a voice communication system in which a relay device and a terminal device are connected over a communication network.[Solution] A voice communication system comprises a terminal device and a relay device which transmit and receive a voice signal with each other over a network. The relay device has a group table. The terminal device transmits, to the relay device, specifying information specifying a group to which the terminal device belongs. When receiving the specifying information from the terminal device, the relay device registers the terminal device in the group table, so as to belong to the designated group. The terminal device transmits, to the relay device, a voice signal destined to the group to which the terminal device belongs. When receiving, from the terminal device, the voice signal destined to the belonging group, the relay device transfers the voice signal to another terminal device belonging to the specified group.