PURPOSE: To attain the diagnois of a common bus of a CPU with use of the output of a voltage comparator by resetting the CPU with a switch and delivering the data of "1" or "0" to each signal line constituting the common bus.

CONSTITUTION: A switch 7 is turned on the reset a CPU1, and the voltage V1 or V0 is set at a prescribed level while a driver 8 is delivering "1" or "0". While a reference voltage generating circuit 9 delivers the high or low level input voltage set by a system 5 while the driver 8 is delivering "1" or "0". A voltage comparator 10 compares the high or low level input voltage with the voltage V1 or V0 of a common bus 4 for diagnosis. Therefore the impedance of the bus 4 has a change when an anomalous arises on a signal line of the bus 4, and the V1 or V0 of each signal line has fluctuation. This fluctuation is detected by the comparator 10.