PURPOSE: To prevent deterioration of battery capacity in a system for automatically stopping/starting an engine at crossing, by stopping the function when the operating amount of vacuum pump for feeding negative pressure exceeds over predetermined level during automatic stoppage of engine.

CONSTITUTION: A negative pressure operation brake booster 20 is coupled to negative pressure port 14 opening to the intake manifold 12 of engine 10 while a motor vacuum pump 26 is coupled through a check valve 22 and conduit 24. Furthermore, a negative pressure detecting switch 28 is coupled to the brake booster 20. An electronic control unit 30 is provided with signals from an idle switch 36 for detecting the full-open state of throttle valve, start switch 38, parking switch 42, car speed sensor 44, engine rotary speed sensor 48, main switch 49, etc. The control unit 30 will feed signals to starter relay 52, igniter 56, injector 58, display 60, battery capacity drop alarm 62 and pump relay 64.