A relaying device, comprising: a network communication unit configured to connect with a communication network and capable of communicating with a plurality of terminal devices via the communication network; a control unit configured to establish a communication session when a calling audio signal is received from the first terminal device, wherein the communication session is participated by the first terminal device and a terminal device of a communication partner designated by the calling audio signal, and an audio signal is mutually transferred between the terminal devices participating in the communication session; a priority information storage unit configured to store priority information of the communication session; wherein the control unit is further configured to establish a plurality of communication sessions including: an individual communication session in which one terminal device and one partner terminal device participate, a group communication session in which the terminal device belonging to a prepared group participates, and a plenary communication session in which all the communicable terminal devices participate; and the priority information storage unit stores the priority information of each of the plurality of the communication sessions; the control unit further comprising a priority determining unit configured to determine the priority of the existing communication session and a new communication session which should be established by the calling audio signal on the basis of the priority information for each of the plurality of communication sessions on receiving the calling audio signal designating the terminal device participating in the existing communication session as the communication partner; the control unit further being configured to make the terminal device participating in the existing communication session resign from the existing communication session and to participate in the new communication session in the case the priority of the new communication session is higher than that of the existing communication session. a network communication unit configured to connect with a communication network and capable of communicating with a plurality of terminal devices via the communication network; a control unit configured to establish a communication session when a calling audio signal is received from the first terminal device, an individual communication session in which one terminal device and one partner terminal device participate, a group communication session in which the terminal device belonging to a prepared group participates, and a plenary communication session in which all the communicable terminal devices participate; and the priority information storage unit stores the priority information of each of the plurality of the communication sessions; the control unit further comprising a priority determining unit configured to determine the priority of the existing communication session and a new communication session which should be established by the calling audio signal on the basis of the priority information for each of the plurality of communication sessions on receiving the calling audio signal designating the terminal device participating in the existing communication session as the communication partner; the control unit further being configured to make the terminal device participating in the existing communication session resign from the existing communication session and to participate in the new communication session in the case the priority of the new communication session is higher than that of the existing communication session.