PURPOSE: To provide a reference mark at a position where the resolution of position setting is improved by increasing the possibility of encountering the reference mark during a search.

CONSTITUTION: The reference mark 43 which is arranged on a surface 40 and can be detected optically consists of a pattern which is encoded according a multibit binary code like a bar code. When an optical sensor 45 is arranged at a certain position as to one predetermined bit, response characteristics of the optical sensor become maximum and are detected by a processor 46, and the surface 40 is moved by a converting device 47 and set at a reference position.