PURPOSE: To accurately detect the position of the titled moving body even in case of a shift in origin by installing a laser photodetector at a previously measured fixed point, and detecting the position and correcting the origin shift of a projection part.

CONSTITUTION: Laser projector 1 and 2 are installed at reference positions A and B and a photodetecting element 4 is installed on the straight line connecting the reference points A and B. Then the optional fixed point P1 is accurately measured by a light wavelength measuring instrument, etc., and its position data is inputted to a real value input device 10. The device 10 converts the input position data into the angle of rotations of the projection parts 1a and 2a, and the angles are stored temporarily. Then a laser photodetector 3 installed on the moving body C is positioned at the fixed point P1 to find and angles α and β of rotation and a position arithmetic unit 14 computes the position of the photodetector 3. Simultaneously, the angle of rotation based upon the position data on the fixed point P1 and the actual angles α and βare compared to calculate and correct errors αe and βe in the angle of rotation. Therefore, even if there is the origin shift, the position of the moving body is accurately detected.