A relay apparatus comprising: a communicator that communicates with a plurality of communication terminals (20, 22, 23, 24, 26, 28) that performs a voice communication; a communication session processor in which a call from a certain communication terminal (20, 22, 23, 24, 26, 28) calling the other at least one communication terminal (20, 22, 23, 24, 26, 28) allows the calling communication terminal (20, 22, 23, 24, 26, 28) and the called at least one communication terminal (20, 22, 23, 24, 26, 28) to be registered as participating terminals, a communication session identified based on session information is established, and transmission of a voice signal from any of the participating terminals in the established communication session allows the voice signal to be transmitted, together with the session information, to the other participating terminal in the same communication session; and a communication monitor that monitors the voice signal in the communication session, wherein the communication session processor notifies the communication monitor of information on the participating terminals in the communication session when the communication session is established, and when the communication monitor is notified of information on a communication terminal (20, 22, 23, 24, 26, 28) in the case of establishing the communication session, the communication monitor individually inputs and records the voice signal transmitted from the communication terminal (20, 22, 23, 24, 26, 28) so that the voice signal is distinguished from the voice signal from the other communication terminal (20, 22, 23, 24, 26, 28), or records the voice signal transmitted to the communication terminal (20, 22, 23, 24, 26, 28) in the communication session, characterized in that the communication monitor sets a virtual device associated with either an input buffer in which the voice signal transmitted from the communication terminal (20, 22, 23, 24, 26, 28) to be monitored, of which the voice signal is recorded, is buffered, or an output buffer in which the voice signal transmitted to the communication terminal (20, 22, 23, 24, 26, 28) to be monitored is buffered, when the communication monitor is notified of information on the communication terminal (20, 22, 23, 24, 26, 28) to be monitored in a case in which the communication session is established, the communication monitor allows the communication terminal (20, 22, 23, 24, 26, 28) to be monitored and the virtual device to establish a virtual communication session, and the communication monitor records the voice signal transmitted to the virtual device in the virtual communication session. a communicator that communicates with a plurality of communication terminals (20, 22, 23, 24, 26, 28) that performs a voice communication; a communication session processor in which a call from a certain communication terminal (20, 22, 23, 24, 26, 28) calling the other at least one communication terminal (20, 22, 23, 24, 26, 28) allows the calling communication terminal (20, 22, 23, 24, 26, 28) and the called at least one communication terminal (20, 22, 23, 24, 26, 28) to be registered as participating terminals, a communication session identified based on session information is established, and transmission of a voice signal from any of the participating terminals in the established communication session allows the voice signal to be transmitted, together with the session information, to the other participating terminal in the same communication session; and a communication monitor that monitors the voice signal in the communication session, wherein the communication session processor notifies the communication monitor of information on the participating terminals in the communication session when the communication session is established, and when the communication monitor is notified of information on a communication terminal (20, 22, 23, 24, 26, 28) in the case of establishing the communication session, the communication monitor individually inputs and records the voice signal transmitted from the communication terminal (20, 22, 23, 24, 26, 28) so that the voice signal is distinguished from the voice signal from the other communication terminal (20, 22, 23, 24, 26, 28), or records the voice signal transmitted to the communication terminal (20, 22, 23, 24, 26, 28) in the communication session, characterized in that the communication monitor sets a virtual device associated with either an input buffer in which the voice signal transmitted from the communication terminal (20, 22, 23, 24, 26, 28) to be monitored, of which the voice signal is recorded, is buffered, or an output buffer in which the voice signal transmitted to the communication terminal (20, 22, 23, 24, 26, 28) to be monitored is buffered, when the communication monitor is notified of information on the communication terminal (20, 22, 23, 24, 26, 28) to be monitored in a case in which the communication session is established, the communication monitor allows the communication terminal (20, 22, 23, 24, 26, 28) to be monitored and the virtual device to establish a virtual communication session, and the communication monitor records the voice signal transmitted to the virtual device in the virtual communication session.