A relaying device, comprising: a network communication unit for communicating with terminal devices via a network; a control unit for controlling communication with the terminal devices; and a storage unit in which a session table is provided, wherein said session table provided in the storage unit for storing a communication session, and the network communication unit configured for receiving an audio signal in the form of a voice packet to which information designating the terminal device of a communication partner is attached in a payload of the voice packet from the terminal device, the control unit configured for registering a communication session made up of the terminal device transmitting the audio signal and the terminal device designated by the designating information attached to the audio signal as participating terminals into the session table when the first voice packet is received from the terminal device without a SIP protocol, the control unit configured to transfer the audio signal received from the participating terminal of the communication session to the participating terminals of the communication session, the control unit configured to mix the audio signals and to transmit the mixed audio signal to the participating terminals when the audio signals are received from two or more participating terminals of the communication session at the same time, and the control unit configured to deregister the communication session from the session table when a predetermined time or more elapses without receiving the audio signal from any of the participating terminals of the communication session regardless of not receiving a termination request of the communication session. a network communication unit for communicating with terminal devices via a network; a control unit for controlling communication with the terminal devices; and a storage unit in which a session table is provided, wherein said session table provided in the storage unit for storing a communication session, and the network communication unit configured for receiving an audio signal in the form of a voice packet to which information designating the terminal device of a communication partner is attached in a payload of the voice packet from the terminal device, the control unit configured for registering a communication session made up of the terminal device transmitting the audio signal and the terminal device designated by the designating information attached to the audio signal as participating terminals into the session table when the first voice packet is received from the terminal device without a SIP protocol, the control unit configured to transfer the audio signal received from the participating terminal of the communication session to the participating terminals of the communication session, the control unit configured to mix the audio signals and to transmit the mixed audio signal to the participating terminals when the audio signals are received from two or more participating terminals of the communication session at the same time, and the control unit configured to deregister the communication session from the session table when a predetermined time or more elapses without receiving the audio signal from any of the participating terminals of the communication session regardless of not receiving a termination request of the communication session. the network communication unit configured for receiving an audio signal in the form of a voice packet to which information designating the terminal device of a communication partner is attached in a payload of the voice packet from the terminal device, the control unit configured for registering a communication session made up of the terminal device transmitting the audio signal and the terminal device designated by the designating information attached to the audio signal as participating terminals into the session table when the first voice packet is received from the terminal device without a SIP protocol, the control unit configured to transfer the audio signal received from the participating terminal of the communication session to the participating terminals of the communication session, the control unit configured to mix the audio signals and to transmit the mixed audio signal to the participating terminals when the audio signals are received from two or more participating terminals of the communication session at the same time, and the control unit configured to deregister the communication session from the session table when a predetermined time or more elapses without receiving the audio signal from any of the participating terminals of the communication session regardless of not receiving a termination request of the communication session.