A novel polypeptide, a polynucleotide encoding this polypeptide, an expression vector comprising this polynucleotide, a cell transfected with the expression vector, an antibody binding to the above polypeptide, a convenient screening method for obtaining an agent for treating joint diseases, and a process for manufacturing a pharmaceutical composition for treating joint diseases are disclosed.The polypeptide is a novel aggrecanase causative of joint diseases (particularly an OA disease).