It is intended to provide a bread-improving agent which shows a highly excellent preservative effect of imparting antifungal properties while sustaining effects of improving the physical properties and taste of bread products. A fermented soybean protein material having been treated with a microorganism belonging to the genus Bifidobacterium is prepared and used in bread products. As a result, itis found out that the above material shows not only an effect of inhibiting the growth of bacteria but also an effect of remarkably inhibiting the growth of fungi which seriously affect the bread qualities. It is furthermore found out that this fermented soybean protein material has effects of imparting excellent physical properties and taste to bread products in addition to the function as a fungal growth inhibitor.