PURPOSE: To stably detect the position of an obstacle in three-dimensional space by photographing a light spot projected to the obstacle with two cameras arranged at a fixed interval, and processing the image of the light spot to determine its position, and continuously carrying out this process.

CONSTITUTION: The first and the second camera A, B are arranged at a fixed interval, and their visual axes are made to intersect mutually at a spot being a reference distance ahead for measuring the width of a camera photograph domain on the crossing face of camera transverse axes. A laser beam is projected to a measured body 1 by a laser projector 2, and converged into a spot-shape. Next luminescence at the laser spot is caught by the cameras A, B, and inputted into an image processing device 3 and binary-processed. In this case, the visual axis of the camera and the central axis of an image plane are made to coincide mutually. Next the spot position on a binary image is measured, and the two-dimensional coordinates of the spot position on the measured body 1 are measured from the above spot position and the previously set up arrangement of the cameras A, B. Three dimensonal coordinates are computed by a personal computer 4 on a basis of two dimensional coordinates.