PURPOSE: To quickly and exactly point out a defective unit by using the own/ other unit separating information that is decided by a unit itself as well as an error status detected by the diagnostic unit itself.

CONSTITUTION: The signal lines EM, EE, etc., which show a fact that its own unit detect some error are connected to a diagnostic unit D from each unit. Each of units including the unit D which report the occurrence of errors owns a means that holds an error status consisting of a 1st flag showing the high probability of a trouble of the unit itself that report the error and a 2nd flag showing the high probability of a trouble of another unit except the one that report its error. Then the error is reported to the unit D in the case the 1st or 2nd flag is set. When an error is produced, the unit D reads and analyzes the error status information to point out a defective unit. In such a way, the defective unit can be pointed out quickly and surely.