A crankshaft is normally and reversely rotated at a constant speed, during which the singular points of crankshaft driving motor current responding to the drive torque change of the crankshaft are detected. A crankshaft rotational angle reference point is set based on the midpoint between the crankshaft rotational positions obtained when, among these singular points, singular points having the same type appear during the respective normal and reverse rotations. Therefore, the crankshaft rotational angle reference point can be reliably set regardless of the variations and errors.