Japanese cultivated soybeans (G.max (L.) Merrill) have been known as protein-rich. However, as the world protein malnutrition problem has been urged to be solved, the development of much higher protein strains is considered to be an indispensable task for soybean breeders. Since the hybridizations of G.max x G.soja (= G.ussuriensis) by Williams (1948) and Weber (1950), G. soja has been regarded as a promising protein gene source in breeding of the G. max varieties with high protein.