PURPOSE: To detect the connection or disconnection of an electric load by detecting a change or a predetermined value or more of the charging or discharging current of a battery.

CONSTITUTION: When an electric load 8 is applied when an engine 2 is idled, a current from a generator 5 or a current from a battery 1 is supplied. Thus, a charging current to the battery 1 is quickly reduced, or a discharging current from the battery 1 flows. This quick change is detected by a current detector 6. On the contrary, when the load 8 is disconnected, a current supplied from the generator 5 is temporarily supplied to the battery 1, and hence a charging current is abruptly increased. This abrupt increase is detected by the detector 6, and a large abrupt decrease of the load 8 is detected. Thus, charging/ discharging current of the battery is detected by a battery current detector to effectively detect that a load is abruptly increased or decreased.