Non-contact type displacement sensors which measure the height of a substrate surface are installed above the substrate in order to hold the upper surface of the substrate at a desired height or to maintain the flatness of the substrate. A substrate mounting device is such that a plurality of grooves and of barriers are provided on the upper surface of a table and air is supplied between the substrate and the table to enable the pressure of air to displace the substrate. In addition, the substrate mounting device has such a structure as to make it possible to deform the substrate into an arbitrary convex-concave shape or to make the substrate flat by feeding back the output of the displacement sensor.