Abstract: An optical system for detecting the shape of a three-dimensional object comprising a collimating lens for converting a laser beam to a parallel beam of light, a scanning optical system which moves the parallel beam incident on the object in a direction normal to the object thereby to scan the object with a scanning beam, a mirror which is located in the vicinity of the scanning beam at a predetermined inclination angle with respect to the object, to reflect the beam reflected by the object in a predetermined direction as a reflection beam, an image forming lens which converges the reflection beam to a point and thereby producing an image of the object, and a first optical detector located at the point at which the reflection beam is converged to detect the converged beam spot.