PURPOSE: To easily detect short-circuit between CPU connection wiring by outputting a pulse of constant period, and shifting a period of each pulse when the abnormality of a CPU is detected.

CONSTITUTION: The CPUs 1 and 2 are connected electrically with each other by connection wiring 3, 4 formed on a substrate. This system is comprised in such a way that the pulse 5 provided with pulse width t of constant period is outputted from the CPU 1, and it is inputted to the CPU 2 passing the connection wiring 3, and also, the pulse 6 provided with the same pulse width t as that of the pulse 5 and whose phase is shifted by 180° from that of the pulse 5 is outputted from the CPU 2, and it is inputted to the CPU 1 passing the connection wiring 4, and furthermore, a synchronizing signal 7 which takes synchronism between the pulses 5, 6 can be sent from the CPU 1 to the CPU 2. The CPU 2 judges that the CPU 1 is operated normally when the pulse width t of the pulse 5 is within a prescribed range, and the CPU 1 judges that the CPU 2 is operated normally when the pulse width t of the pulse 6 is within the prescribed range.