In an apparatus for detecting a position of a substrate in a film thickness measuring apparatus, an image pickup area on a substrate by an image pickup device of an imaging part is rotated by a rotation mechanism of a position detecting part. With this operation, an image of an edge of the substrate can be easily picked up at a plurality of image pickup positions around a central axis to detect a position of the substrate, without providing a mechanism for rotating a stage holding the substrate. As a result, it is possible to suppress upsizing of a structure for detecting a position of the substrate in the film thickness measuring apparatus and to achieve high-speed and high precision-position detection of the substrate, as compared with a conventional apparatus where a mechanism for rotating a substrate is provided above a mechanism for moving the substrate in a horizontal direction.