Summer crop plants grown in rain-fed fields suffer from drought during a hot and relatively dry summer after the rainy season called "Baiu" in Japan. Since crop plants might develop vigorous shoots with poorly developed root systems during the rainy season, they might suffer from water deficits in the summer, even when soil moisture depletes on the surface of the soil. In order to confirm this, the effects of pre-flowering soil moisture deficits on dry matter production and ecophysiological characteristics thereafter were investigated in soybean plants. Under deficient soil moisture conditions after flowering, higher dry matter production and higher grain yield were attained in the plants grown under deficient soil moisture before flowering (D-plot) than in the plants grown under sufficient soil moisture (W-plot). Large net assimilation rate (NAR) was responsible for high dry matter production in the plants of the D-plot. High NAR in the plants of the D-plot resulted from (1) a root system that was well developed in deep soil layers, (2) their capacity to absorb much soil water, especially in deep soil layers, (3) maintenance of a high leaf water potential and, therefore, a high photosynthetic rate during daytime and (4) a delay in the decrease in photosynthetic rate due to senescence. It was concluded that improved cultivation for developing root systems, such as drainage in the rainy season, as well as irrigation in midsummer, will be important in the cultivation of field summer crop plants in Japan.