A guide member and a slide are disposed such as to be opposed to each other through a bearing clearance therebetween. Air is supplied to the bearing clearance through a main pipe, a branch pipe and an orifice provided in the slide. Shape and size of each portions of the main pipe, the branch pipe and the orifice are determined such that Reynolds number becomes 2000 or less in any of the portions of the main pipe, the branch pipe and the orifice so that a turbulent flow is not generated in air flow approaching the bearing clearance.