PURPOSE: To prevent a reference point for detecting the position of a mobile object from being erroneously recognized.

CONSTITUTION: The mobile object is traveled within the range for which plural areas surrounded with four reference points including two reference points mutually shared by adjacent areas are set. At an out-of-area reference point azimuth angle arithmetic part 16 consisting of the control part of the mobile object, the azimuth angle of the reference point in the area adjacent to the area, where the mobile object is traveled at present, is calculated. An azimuth angle detecting part 12 detects the azimuth angle of the reference point corresponding to the output signal of a scanner 2. When the actually detected azimuth angle is equal to the calculated azimuth angle, the actually detected azimuth angle is excluded by a buffer 13 corresponding to the signal of a comparing part 17. A move detecting part 18 detects that the mobile object is moved to the adjacent area, outputs a detecting signal to a coordinate storage part 15 or a self position arithmetic part 14 and transforms a coordinate system.