A transmitting and receiving system for transmitting and receiving a data signal and a control signal between an operation system unit and a standby system unit, comprising: a multiplexer (15, 15-1, 15-4) included in the operation system unit; a demultiplexer (15, 15-2, 15-3) included in the operation system unit; a multiplexer (25, 25-1, 25-4) included in the standby system unit; a demultiplexer (25, 25-2, 25-3) included in the standby system unit; a circuit configured to perform a parallel/serial conversion on a data signal and a control signal using the multiplexer (25, 25-1, 25-4) of the standby system unit to multiplex the signals in order to transmit, from the standby system unit to the operation system unit, the signals as a serial received signal, and a further circuit configured to perform a serial/parallel conversion on the signal using the demultiplexer (15, 15-2, 15-3) of the operation system unit to demultiplex the serial received signal, the data signal and the control signal being to be received by the standby system unit from an equipment connected to the standby system unit and being to be transmitted to a receiving device (UART, 14) in the operation system unit; and a circuit configured to perform a parallel/serial conversion on a data signal and a control signal using the multiplexer (15, 15-1, 15-4) of the operation system unit to multiplex the signals in order to transmit, from the operation system unit to the standby system unit, the signals as a serial signal to be transmitted, and a further circuit configured to perform a serial/parallel conversion on the signal using the demultiplexer (25, 25-2, 25-3) of the standby system unit to demultiplex the received signal, the data signal and the control signal being to be input from a transmitting device (UART, 14) of the operation system unit and being to be transmitted to the equipment connected to the standby system unit. a multiplexer (15, 15-1, 15-4) included in the operation system unit; a demultiplexer (15, 15-2, 15-3) included in the operation system unit; a multiplexer (25, 25-1, 25-4) included in the standby system unit; a demultiplexer (25, 25-2, 25-3) included in the standby system unit; a circuit configured to perform a parallel/serial conversion on a data signal and a control signal using the multiplexer (25, 25-1, 25-4) of the standby system unit to multiplex the signals in order to transmit, from the standby system unit to the operation system unit, the signals as a serial received signal, and a further circuit configured to perform a serial/parallel conversion on the signal using the demultiplexer (15, 15-2, 15-3) of the operation system unit to demultiplex the serial received signal, the data signal and the control signal being to be received by the standby system unit from an equipment connected to the standby system unit and being to be transmitted to a receiving device (UART, 14) in the operation system unit; and a circuit configured to perform a parallel/serial conversion on a data signal and a control signal using the multiplexer (15, 15-1, 15-4) of the operation system unit to multiplex the signals in order to transmit, from the operation system unit to the standby system unit, the signals as a serial signal to be transmitted, and a further circuit configured to perform a serial/parallel conversion on the signal using the demultiplexer (25, 25-2, 25-3) of the standby system unit to demultiplex the received signal, the data signal and the control signal being to be input from a transmitting device (UART, 14) of the operation system unit and being to be transmitted to the equipment connected to the standby system unit.