PURPOSE: A method of producing functional soy sauce(sarijang) using Rhynchosia volubilis, sulfur fed duck, garlic, bamboo salt and herbs containing Ulmi radicis cortex, Lonicerae Flos and the like as a main material is provided. The product stimulates blood production, prevents diseases such as mysterious disease, adult disease, cancer and the like while removing impurities in blood. CONSTITUTION: Rhynchosia volubilis is soaked in water, heated at 100deg.C for 2 to 3hr and allowed to settle by its own heat at about 90deg.C for 60 to 90min to give improved fermented soybeans(meju). The fermented soybeans are fermented at 45 to 50deg.C in an ondol room and heated with water for 2 to 3hr after drying to give backbitang. The backbitang is mixed with 40 to 55% by weight of bamboo salt, 15 to 30% by weight of sulfur fed duck, 10 to 30% by weight of Ulmi radicis cortex, 5 to 20% by weight of garlic and a remaining portion of other herbs, extracted, squeezed and added with bamboo salt. The other herbs contain Herba T
