The invention provides a chondroitin sulfate synthesis promoter useful for the treatment of diseases such as articular disease and discopathy. The chondroitin sulfate synthesis promoter contains, as an active ingredient, chondroitin sulfate glucuronyltransferase protein and/or chondroitin sulfate N-acetylgalactosaminyltransferase-1 protein, or a gene encoding the enzyme protein(s).