PURPOSE: To prepare "natto" (fermented soybean) containing "wakame" seaweed and having mixed taste and flavor of the "wakame" and the "natto", wherein the "wakame" contained therein is slimy and has green color characteristic to "wakame", by fermenting seasoned and immediately cooled "wakame" together with soybeans using Bacillus natto.

CONSTITUTION: Raw "wakame" or reconstituted dried "wakame" is cut to breadth of 2mm and length of about 10mm, seasoned with boiling seasoning liquid prepared from soy sauce, "mirin" (a sweet sake used as seasoning), dried bonito, a chemical seasoning, etc., drained, cooled preliminary with e.g. an electric fan, and cooled in a refrigerator to 2W5°C. On the other hand, steamed soybeans are inoculated with Bacillus natto, and mixed with 10W15wt% of the seasoned and cooled "wakame" prepared above. The mixture is fermented by conventional method.