A communication system comprising: a first controller connected to a first communication terminal; a second controller connected to a second communication terminal; and a relaying apparatus configured to establish a session for transmitting content data between the first communication terminal and the second communication terminal, the first controller comprising: a request accepting unit that accepts a request to start communication between the first communication terminal and the second communication terminal; a first restricting unit that imposes a restriction so that the first communication terminal does not start any communication with another communication terminal other than the second communication terminal in response to the request; and a first transmitter that transmits, to the second controller, terminal identification information for identifying the second communication terminal, the second controller comprising: a second restricting unit that imposes, based on the terminal identification information transmitted from the first controller, a restriction so that the second communication terminal does not start any communication with another communication terminal other than the first communication terminal in response to the request; and a second transmitter that transmits, to the first controller, restriction information indicating that the restriction on the second communication terminal (lObd) is completed, and the first controller transmitting, to the relaying apparatus, a request to establish a session for transmitting content data between the first communication terminal and the second communication terminal after the restriction on the first communication terminal and the restriction on the second communication terminal are completed, wherein when a new request to start communication between communication terminals including the first communication terminal or the second communication terminal is received while the restriction by the first controller and the restriction by the second controller are imposed, the first controller and the second controller each reject the new request. a first controller connected to a first communication terminal; a second controller connected to a second communication terminal; and a relaying apparatus configured to establish a session for transmitting content data between the first communication terminal and the second communication terminal, the first controller comprising: a request accepting unit that accepts a request to start communication between the first communication terminal and the second communication terminal; a first restricting unit that imposes a restriction so that the first communication terminal does not start any communication with another communication terminal other than the second communication terminal in response to the request; and a first transmitter that transmits, to the second controller, terminal identification information for identifying the second communication terminal, a request accepting unit that accepts a request to start communication between the first communication terminal and the second communication terminal; a first restricting unit that imposes a restriction so that the first communication terminal does not start any communication with another communication terminal other than the second communication terminal in response to the request; and a first transmitter that transmits, to the second controller, terminal identification information for identifying the second communication terminal, the second controller comprising: a second restricting unit that imposes, based on the terminal identification information transmitted from the first controller, a restriction so that the second communication terminal does not start any communication with another communication terminal other than the first communication terminal in response to the request; and a second transmitter that transmits, to the first controller, restriction information indicating that the restriction on the second communication terminal (lObd) is completed, and a second restricting unit that imposes, based on the terminal identification information transmitted from the first controller, a restriction so that the second communication terminal does not start any communication with another communication terminal other than the first communication terminal in response to the request; and a second transmitter that transmits, to the first controller, restriction information indicating that the restriction on the second communication terminal (lObd) is completed, and the first controller transmitting, to the relaying apparatus, a request to establish a session for transmitting content data between the first communication terminal and the second communication terminal after the restriction on the first communication terminal and the restriction on the second communication terminal are completed, wherein when a new request to start communication between communication terminals including the first communication terminal or the second communication terminal is received while the restriction by the first controller and the restriction by the second controller are imposed, the first controller and the second controller each reject the new request.