PURPOSE: To easily supervise and control the operational state of each constituting computer by outputting operational state signals showing the operational states from every equipment coupled to an n:n transmission system and supervising the operational state of the other equipment according to the signal in each equipment.

CONSTITUTION: A computer 1 is operated as a main system and a computer 2 is operated as a slave system. The operational state signal of the computer 1 is stored through an n:n transmission system 5 to areas 23 of respective shared memories 8, 9 and 10, and the operational state signal of the computer 2 is stored through the transmission system 5 to areas 24 of the respective memories 8,9 and 10. When the computer 1 breaks down, the change of alternating the operational state signal to be outputted from the computer 1 is stopped. Then, the computer 2 immediately detects the stop of this change from the 23 of the memory 9 after the lapse of supervisory time, recognizes the stop of the computer 1 switches the operational state from the slave system to the main system and outputs the operations state signal after changing it. A controller 6 recognizes it from values stored in the 23 and 24 of the memory 10 that the computer 1 stops the operation and that the operation is started after the computer 2 is changed from the slave system to the main system, and the control output value of the computer 2 is inputted so as to continue the control of the controller.