An ignition unit has a superimposed voltage generation circuit that feeds, between electrodes of an ignition plug, a superimposed voltage of the same direction as a discharge voltage, and in an operation range wherein an engine rotation speed is equal to or lower than a given speed and an engine load is equal to or lower than a given load, feeding of the superimposed voltage is carried out. The panel has a button, an indicator hole and an emergency ejecting hole. The energy of an impact can thus be absorbed as the intermediate layer is stretched into a bag shape and the inner cover layer breaks off progressively. The rim of the flywheel is toothless. The case 2 includes a plurality of holes 3 that penetrate from the outside to the inside of the case 2, and edges of the holes 3 include projections 3a that protrude from the inner surface of the case 2 towards the inside thereof.