A screw shaft, a nut, and a conduit member are made of metal material, and a solid lubrication film is coated on inner walls of thread grooves thereof and a ball circulation path of the conduit member. Where d1 is the inner diameter of the ball circulation path of the conduit member, d2 is the groove diameter of the screw shaft, d3 is the diameter of each ball, and h1 is a clearance between the screw shaft and the end portion of the thread groove (as well as the end portion of the conduit member), respective members are arranged so as to satisfy relations of (.vertline.d1-d2.vertline./d3).ltoreq.1/40 and h1/h3.ltoreq.1/40. As a result, it is possible to provide a ball screw mechanism in which film-transfer lubrication is achieved between the members and the durability and precision are improved.