To perform height measurement on minute projections by using a simple constitution based on a confocal method and in a short time by making smaller the moving distance of a focal position in an optical system.

While moving the focal position of an optical unit by vertically moving an objective lens 24 of the optical unit by means of a piezo element 50, the surface of a substrate 1 mounted on a table is vertically illuminated with white light. Reflected light from the surface of the substrate 1 is condensed while the reflected light condensed is received through a pinhole. The heights of minute projections on the surface of the substrate 1 are detected from a change in the intensity of the reflected light received. The height of a substrate mount surface of the table for mounting the substrate 1 thereon is previously detected by using a height sensor 30. Based on the result of detection, the height of the optical unit is adjusted with respect to the surface of the substrate 1 mounted on the table. The moving of the focal position of the optical unit can be started at a constant distance from the surface of the substrate 1 at all times even if the height of the surface of the substrate 1 is uneven owing to undulations of the mount surface of the table.