An object of the present invention is to provide a method for preventing diseases in weaned piglets, which relieves the stress of postweaning piglets and enhances the immunostimulating ability in order to prevent the death due to colibacillosis such as edema disease in postweaning piglets and to increase the rate of raising.Pig diseases such as colibacillosis, particularly diarrhea, can be prevented by administering a Grifola-derived substance selected from one or more of dried Grifola, dried Grifola powder and a Grifola extract to pigs in weaning period, so as to solve the problems of the present invention.