A vehicle speed sensor detects a rotational speed of a rear wheel. A throttle valve is positioned downstream of an intake pipe. An idle speed control device controls the amount of air flowing through the intake pipe to perform an idle speed control. An electronic control unit (ECU) detects an abnormality of the idle speed control (ISC) device based on an intake pipe pressure. The ECU inhibits a rotational speed of the engine from increasing when an abnormality of the ISC device is determined. Precise detection of vehicle speed and suppression of increase of engine rotational speed resulting from an ISC abnormality is thereby achieved.