To make clear the digestion process of soybean proteins during miso fermentation, water-insoluble proteins prepared at various stages of miso fermentation have been investigated by polyacrylamide gel electrophoresis using sodium lauryl sulfate (SDS PAGE) and gel filtration in 6M guanidine hydrochloride (GuHCl). On the gel of SDS PAGE, soybean proteins showed obscure bands during the initial 10 days of fermentation and newly formed bands of lower molecular weight (about 1×104) appeared. The similar tendency was also observed in the elution patterns from 6M GuHCl Sepharose 6B column. Therefore, it was concluded that soybean proteins could undergo the digestion within several days of fermentation and the water-insoluble proteins of miso were resistant to the further hydrolysis by the koji enzymes.