A device is provided with which it is made possible to rapidly determine that an internal combustion engine is in a stop state and to prevent an erroneous determination of starting. In a stop determination device provided with a detection unit that detects the rotation direction of the crankshaft of an internal combustion engine, it is determined that the internal combustion engine is in a stop state, in the case where a signal from the detection unit is not inputted for a predetermined time or in the case where a rotation-direction signal from the detection unit continuously reverses thrice or more times. Additionally, in the case where, from the detection unit, a signal of the same rotation direction is continuously inputted twice or more times, a determination that an internal combustion engine is in a stop state is cancelled.