PURPOSE: To improve the reliability of the dual operation control, by making a single and dual mixed operation control in respect to the operation control between a dual operation object terminal and a computer.

CONSTITUTION: The signal transmitted from a communication terminal is received by a transmitting/receiving circuit 23 of a dual controller 2 and is sent to a communication mode adding circuit 24. The circuit 24 checkes whether CPUs 1 can transmit and receive the signal or not, and the CPU 1 which can transmit the signal is determined, and a signal indicating the single mode or the dual mode in accordance with the check result is added to the signal from the communication terminal, and this signal is sent to a transmitting/receiving circuit 13 of the CPU 1 through a transmitting/receiving circuit 21. The signal received by the circuit 13 is sent to a multiple processing circuit 14, and the operation mode signal added by the circuit 24 is discriminated; and as the result, in case of the single mode, the signal sent from the device 2 is transferred to the other computer, and processings are executed in respective computers, and the response is outputted from the computer which can transmit the signal. In case of the dual mode, output signals of both computers are compared with each other in a collating circuit 22; and if they coincide with each other, the output signal is outputted to the terminal. In figure, additional characters A and B indicate systems A and B respectively.