The present inventors discovered that knockout mice whose S1-5 gene function is lost develop age-related diseases or symptoms. Histological analysis in such knockout mice revealed that bone mineral content, bone mineral density, and bone strength were decreased, and the number of osteoclasts in bone tissues was increased. Analysis of osteoclast-forming ability using bone marrow cells derived from the knockout mice revealed that osteoclast-forming ability is enhanced and osteoclasts are larger in the knockout mice than in wildtype mice. When purified S1-5 protein was added to this in vitro system, osteoclast-forming ability was inhibited.