PURPOSE: To display a diagnostic result by using a single annunciat-means which indicates the diagnostic result by connecting a control unit for a system to be diagnosed to the annunciating means through a system selecting means.

CONSTITUTION: A display device 1 as the annunciating means which displays the diagnostic result is connected to the system selecting means 7 through a harness 8 for one system. The means 7 is connected to control units 2, 3, 4, and 5 through a harness 6 and the units 2, 3, 4, and 5 to be diagnosed are selected among the units 2, 3, 4, and 5 and connected to the harness 8. Thus, the diagnostic results of plural systems are displayed by using the single annunciating means and signal lines between the annunciating means and system selecting means are reduced to the number of lines for one system.