An ignition device for a personal watercraft engine, which allows mounting of a four-cycle engine on the personal watercraft and prevents occurrence of leakage of current. One image is deformed to the coordinate space of the other image, and the differences between the intensity values of the other image as compared to the mapped image are evaluated as measures of ventilation. The absorbent layer forming the flat absorber is formed with a plurality of small recesses which open in the surface of the absorbent layer and extend into the absorbent layer. A conduit delivers gas to the plenum.