The present invention provides compositions and methods for treating a joint disease containing as an active ingredient thereof a monovalent metal salt of alginic acid for which the endotoxin level thereof has been lowered to an extent that does not substantially induce inflammation or fever. As a result, it is possible to provide a composition for treating a joint disease which has the effects of protecting cartilage from mechanical irritation, inhibiting degenerative changes in cartilage caused by wear and inflammation, repairing cartilage injuries, suppressing inflammation and pain of joint tissue, inhibiting degeneration of synovial tissue, and inhibiting osteochondral destruction.