The cultivation of early maturing soybean is rarely seen in southwest of Japan. A considerable reason is the problem of low seed quality caused by high temperature and humidity of summer during seed maturation. The objective of this study was to evaluate the influence of precipitation during harvest time on the seed quality in early maturing soybean cultivars. Harvested dry pods of 63 cultivars matured in different season (summer and autumn) were soaked in distilled water for 0, 2, 4 and 6 days. The seed moisture content and the deterioration of seed quality after soaking were investigated. The results were as follows. In the pods of summer harvest, the seed moisture content, and the rate of damaged seeds (contained the seeds of decay, injury and seed coat cracking, etc.) were greatly higher than those of harvested in autumn by the pods soaking treatment. The seed damage occurred dramatically when the seed moisture content was more than 50%. Moreover, the water absorption of seeds through pod shell seemed to be different among cultivars since the low seed moisture content cultivars in summer harvest had shown also low seed moisture content in autumn harvest. It is suggested that the deterioration of seed quality by precipitation during harvest time could be relieved by the selection of cultivar.