To correctly select the last MDSEL (mode select) switch state even if a MDSEL switch is changed to a contact of a failed line of A, B systems when one line of the system failed.

CPU 21, 22 of A, B systems read the input state of contacts 11, 12, 13 of a MDSEL switch 10 and write it into a shared memory 23. The CPU 21 of A system changes the data of A, B systems memorized in the shared memory 23 and selects the contact state of stop/idle/fly (ON/OFF/MISMATCH) from the data of both systems and then determines the last MDSEL switch state from the selected contact state of stop/idle/fly. As is the case with A system, the CPU 22 of B system selects the contact state of stop/idle/fly from the data of both systems which are memorized in the shared memory 23 and then determines the last MDSEL switch state from the selected contact state.