The light emitted from a focal position-detecting light source 38 and reflected from a reflector 12 is projected onto a screen 341, and a projected image on the screen 341 is detected by a focal position-detecting camera 35. Based on this projected image, the focal position-detecting light source 38 is moved by a position adjusting system 37 so as to place the focal position-detecting light source 38 at a focal position of the reflector 12, and the position of the focal position-detecting light source 38 at this time is detected as the focal position of the reflector 12. When the focal position of the reflector 12 is detected, the projected image on the screen 341 detected by the focal position-detecting camera 35 is used, and, therefore, the conventional measurement by a three dimensional measuring instrument is made unnecessary, thereby improving the working efficiency.