Provided is a position detecting device that is capable of precisely detecting the coordinates of the center of a disc-shaped substrate from image data captured by one camera, calculating the amount of positional misalignment of the disc-shaped substrate on a support member as the disc-shaped substrate is being transported during processing, and correcting the position such that the substrate can be located at the precise loading position. Isosceles right triangles, each having the radius of the disc-shaped substrate as one side, are generated from the coordinates of two places on edge data extracted from the image data and the radius on the edge data, and the coordinates of the center position of the disc-shaped substrate are detected by using the Pythagorean theorem.