A reverse rotation preventive device wherein an engine can be restarted without once stopping the vehicle even in the case where a reverse rotation preventive function is operated in response to momentary locking of a rear wheel. A plurality of reluctors are arranged at regular intervals, exclusive of a toothless part at one location with a crank pulse rotor rotated synchronously with a crankshaft of the engine, and a first and second pulse generators each of which outputs a pulse signal corresponding to the interval of the reluctors. Reverse rotation preventive means inhibits ignition of a spark plug by operating the reverse rotation preventive function when the interpulse time of the pulse signal exceeds a predetermined time and cancels the inhibition of the ignition when it is detected that a crankshaft is in normal rotation after the ignition is inhibited by the operation of the reverse rotation preventive function.