Summer buckwheat (Fagopyrum esculentum Moench. cv. Kitawasesoba, summer ecotype) was cultivated from April to June in 1998 in the drained paddy fields of two experimental sites:Hojo city, in the plain area facing the Inland Sea of Japan (Setonaikai), and Honjo village, in the intermediate mountainous area of Kyushu. Two types of experimental fields were prepared on both sites:a field left fallow during winter (fallow plot) and that on which Chinese milk vetch (Astragalus sinicus L.) was cultivated in winter as green manure (Chinese milk vetch plot). In 1997, a Chinese milk vetch plot was also provided in Hojo city. Because the precipitation during summer buckwheat cultivation in 1997 was low, grain yield exceeded 250 g / m2. Because of abundant rainfall around seeding time, however, the establishment percentage was very low and plant growth was markedly poor in 1998. The depression of the establishment of seedings and growth were severer in the fallow plot than in the Chinese milk vetch plot. Grain yield in the fallow plot was nearly zero, while it was 112g / m2 (47% of 1997) in the Chinese milk vetch plot. In Honjo village, the amount of rainfall during the period from flowering to maturing in 1998 was 2.76 times greater than in a normal year. Subsequently, because of the decreased number of grains per plant, grain yields in both plots showed low levels, such as 219g / m2 in the Chinese milk vetch plot and 143g / m2 in the fallow plot. As mentioned, the depression in grain yield caused by excessive soil moisture around seeding time or after flowering period was demonstrated to be severer in the fallow plot that in the Chinese milk vetch plot. These results suggest that Chinese milk vetch as green manure is useful for summer buckwheat cultivation and that excess moisture injury to summer buckwheat is considered to be reduced by the use of Chinese milk vetch.