PURPOSE: To reduce the generation of blind spots even when a to-be-measured object includes large projections and recesses, by condensing two reflecting lights generated so as to hold a laser beam projected at right angles to the object separately by two optical systems.

CONSTITUTION: A laser beam 5 is produced by a laser 1, a collimator lens 2 and a focus-adjusting lens 3, and cast at right angles to a to-be-measured object 4. The irregularly reflected light is reflected as reflecting lights 6, 9 symmetric to each other with respect to the incident beam 5. The reflecting light 6 reaches a position detecting element 8, while the reflecting light 9 comes to a position detecting element 11. When a reflecting point on the object 4 is moved up and down, image points of the position detecting elements 8, 11 are moved. The surface displacement is measured from the moving amount of the image points based on the principle of the trigonometrical survey. If the surface of the object 4 is considerably rough and a condensing optical system of one of the position detecting elements 8, 11 becomes unable to measure, the other position detecting element works to measure the surface. Accordingly, a blind spot is prevented from being generated.