Even if the engine is started from any crank angle position, it is possible to correctly determine the rotational direction of a crankshaft, so that fuel injection or ignition can be stopped when the crankshaft is rotating in the reverse direction. A measurement member has a plurality of angular position detection portions arranged at equal intervals in a circumferential direction of the crankshaft and a plurality of reference position detection portions at which a part of the angular position detection portions is missing. A crank angle sensor is arranged near the measurement member for generating a crank angle signal representative of the rotational position of the crankshaft. A period detector detects periods of pulses of the crank angle signal. A reference position determiner determines a plurality of reference positions based on the signal periods. A counter counts the pulses of the crank angle signal. A rotational direction determiner detects the rotational direction of the crankshaft from the number of pulses counted between a plurality of reference positions.