Seasonal observations of the culm invasion and the severity of rice stem rot by the causual fungus Leptosphaeria salvinii Cat. on the four rice varieties (early maturity v.: Rikuu No. 132 and Aikoku, Lates ones : Asahi and Norin No. 18) were studied. The stage of culm penetration of the fungus into both early and late maturity varieties rice plants which were cut off their leaf blades is faster about ten or twenty days than not treated ones, and the severity of stem rot also increased by the treatment, leaf cutting. Culms of early maturity varieties were penetrated by the fungus ealier than those of late ones even in comparion with the same growth stage, and the severity of the former varieties is heavier than the latter.