An apparatus is provided with a light-sending optical system which makes first light from a first pattern and second light from a second pattern incident to a predetermined surface to project intermediate images of the first and second patterns onto the predetermined surface respectively; a light-receiving optical system which guides the first and second light reflected on the predetermined surface, to first and second observation surfaces to form observation images of the first and second patterns, respectively; and a detecting section which detects position information of the observation images of the first and second patterns on the first and second observation surfaces respectively and which calculates a surface position of the predetermined surface, based on the position information. The light-sending optical system projects the intermediate image of the second pattern as an inverted image in a predetermined direction relative to the intermediate image of the first pattern.