To additionally provide an oil storage device via the utilization of a space not utilized fully in a circulation block by providing a hole in the space not utilized effectively for circulating a ball in the circulation block, and providing an oil hole connecting this hole to a ball turning path.

A hole 18 for storing grease is formed between the base plate 10 of a ball circulation seat 1, an outer ring 11, and an inner ring 12. Grease shunt leg guide groups 162, 163 are provided on both sides of each of positioning pinholes 16 used for coupling the ball circulation seat 1 and a cover respectively. The grease shunt leg group 162 penetrates a ball turning path 17, and an oil immersion hole 161 is formed between the ball turning path 17 and positioning pinhole 16. The oil immersion hole 161 has a function to inject the grease stored in the hole 18 into the ball turning path 17.