Abstract: A bone-related protein named OSF-8 which is obtained from bone tissue of a mammal including mouse or human, and a process for its production. This protein is a novel naturally occurring mammal protein which belongs to a group of sulfatases. OSF-8 degrades the sulfate groups of the sugar chains of proteoglycan, which makes up the cartilage matrix, during the replacement of cartilage into bone tissue at the stage of osteogenesis. OSF-8 can be used as an agent for treating bone metabolic diseases, and its high organ specificity for bones enables its use as a diagnostic reagent for bone metabolic diseases.