A voice communication system comprising: a) a relaying device and a plurality of terminal devices (14, 24, 34) communicating with each other over a communication network (17, 21, 31), wherein b) the terminal device (14, 24, 34) comprises b1) a voice transmission unit transmitting a calling voice signal attached information specifying a terminal device (14, 24, 34) of a communication partner to the relaying device and b2) a mode setting unit for setting the own terminal device (14, 24, 34) to a particular mode, c) the relaying device comprises c1) a session management unit forming a session of voice communication when the calling voice signal is received from the terminal device (14, 24, 34), wherein the terminal device (14, 24, 34) and a terminal device (14, 24, 34) specified by the calling voice signal participate in the session, c2) a voice transfer unit mutually transferring the voice signal among the terminal devices (14, 24, 34) participating in the session, and c3) a priority information storage unit storing a priority information pertaining to the session, wherein d1) when the calling voice signal specifying the terminal device (14, 24, 34) participating in an existing session as a communication partner is received from the terminal device (14, 24, 34) not set to the particular mode, the session management unit decides a priority between the existing session and a new session to be formed by the current calling voice signal in accordance with the priority information, d2) the session management unit removes the terminal device (14, 24, 34) participating in the existing session from the existing session and makes said terminal device (14, 24, 34) participate in the new session when the priority of the new session is higher than the priority of the existing session, and d3) when a specific calling voice signal specifying the terminal device (14, 24, 34) participating in the existing session as a communication partner is received from the terminal device (14, 24, 34) set to the particular mode, the session management unit removes the terminal device (14, 24, 34) participating in the existing session from the existing session and makes said terminal device (14, 24, 34) participate in a session to be formed by the specific calling voice signal regardless of the priority of both sessions. a) a relaying device and a plurality of terminal devices (14, 24, 34) communicating with each other over a communication network (17, 21, 31), wherein b) the terminal device (14, 24, 34) comprises b1) a voice transmission unit transmitting a calling voice signal attached information specifying a terminal device (14, 24, 34) of a communication partner to the relaying device and b2) a mode setting unit for setting the own terminal device (14, 24, 34) to a particular mode, b1) a voice transmission unit transmitting a calling voice signal attached information specifying a terminal device (14, 24, 34) of a communication partner to the relaying device and b2) a mode setting unit for setting the own terminal device (14, 24, 34) to a particular mode, c) the relaying device comprises c1) a session management unit forming a session of voice communication when the calling voice signal is received from the terminal device (14, 24, 34), wherein the terminal device (14, 24, 34) and a terminal device (14, 24, 34) specified by the calling voice signal participate in the session, c2) a voice transfer unit mutually transferring the voice signal among the terminal devices (14, 24, 34) participating in the session, and c3) a priority information storage unit storing a priority information pertaining to the session, wherein c1) a session management unit forming a session of voice communication when the calling voice signal is received from the terminal device (14, 24, 34), wherein the terminal device (14, 24, 34) and a terminal device (14, 24, 34) specified by the calling voice signal participate in the session, c2) a voice transfer unit mutually transferring the voice signal among the terminal devices (14, 24, 34) participating in the session, and c3) a priority information storage unit storing a priority information pertaining to the session, wherein d1) when the calling voice signal specifying the terminal device (14, 24, 34) participating in an existing session as a communication partner is received from the terminal device (14, 24, 34) not set to the particular mode, the session management unit decides a priority between the existing session and a new session to be formed by the current calling voice signal in accordance with the priority information, d2) the session management unit removes the terminal device (14, 24, 34) participating in the existing session from the existing session and makes said terminal device (14, 24, 34) participate in the new session when the priority of the new session is higher than the priority of the existing session, and d3) when a specific calling voice signal specifying the terminal device (14, 24, 34) participating in the existing session as a communication partner is received from the terminal device (14, 24, 34) set to the particular mode, the session management unit removes the terminal device (14, 24, 34) participating in the existing session from the existing session and makes said terminal device (14, 24, 34) participate in a session to be formed by the specific calling voice signal regardless of the priority of both sessions.