A method of producing riboflavin by culturing riboflavin-producing microbes in a culture medium using a plant oil or an animal oil as a carbon source, forming and accumulating riboflavin therein and collecting riboflavin therefrom, wherein a carrier of a clay mineral having oil-adsorbing property, a chemically treated product thereof or a calcium compound is made present in the culture medium. The riboflavin is produced maintaining a high yield and at an increased production rate at a low cost without requiring cumbersome operations for concentrating and recovering the riboflavin. It is further allowed to recover the riboflavin by effectively utilizing the waste plant oil or the waste animal oil that is to be disposed of.