Disclosed is a therapy which is for preventing or treating cartilage damage and diseases associated with cartilage damage, such as arthritides and osteoarthritis, and utilizes a more effective and more safe medicinal agent. Specifically disclosed are a cartilage production promoter, a glucosaminoglycan and/or proteoglycan production promoter, and a prophylactic or therapeutic agent for diseases associated with cartilage damage, each of which comprises fucoidan as an active ingredient.