To diagnose the abnormality of power source of respective micro- computers by that plural micro-computers watch the supply power source voltage of the micro-computer which is a communication partner, respectively.

Micro-computers 12, 14 mutually exchange and cooperate and process the run pulse data as a watch data per a prescribed term through a communication circuit 26 and watch mutual motion. The input terminal and output terminal of a voltage regulator 16 are connected to the built in A/D converter 28 of the micro-computer 14 respectively and the terminal of the regulator 18 is connected to A/D converter 29 alike and the detection signals outputted by various kinds of sensors are supplied into an input port circuit CPU judges based on a control program whether the voltage of the power source of a battery is abnormal or not and in the abnormal case, a power source line open abnormal diagram indication is carried out. Therefore, as the mutual motion watch is carried out by not only the power source abnormality watch of the micro-computer of the communication partner but also the communication of a watch data, the existence/absence of the abnormality of the micro-computer itself can be diagnosed.