In an idle stop vehicle, a starter has separately an electromagnetic switch for energizing a motor and a pinion control solenoid for engaging a pinion gear with a ring gear. A coil of the solenoid is energized by a battery through a relay. For starting an engine, an ECU turns on the relay and the electromagnetic switch so that the starter cranks an engine. During engine operation, the ECU turns on only the relay while maintaining turn-off of the electromagnetic switch. If a voltage of a current supply path between the relay and the coil is lower than a predetermined threshold value, the ECU prohibits automatic idle stop by determining that the current supply path to the coil has abnormality.