PURPOSE: To suppress heating of a ball screw properly by controlling the feed amount of refrigerant based on the rotating amount of the ball screw and also cool the heating part of the ball screw directly and efficiently by spouting the refrigerant at a contact part between the nut and screw shaft of the ball screw.

CONSTITUTION: A mixer 15 to generate the mixed refrigerant of compressed air and lubricating oil is provided in a refrigerant feeding circuit 8. Also a refrigerant path 20 to flow the mixed refrigerant is provided in a nut 2 and a mobile body 4, and the mixed refrigerant is spouted from the inside of the nut 2 to a contact part between the nut 2 and a screw shaft 3. Then, based on a rotating amount reference value for each unit time of the ball screw 1, an opening and closing selector valve 13 is controlled and, based on the rotating amount for each unit time, the opening degree of a flow control valve 14 is controlled by a NC controller 10.