Two light beams are incident on the same position on a surface of an object (105) to be measured obliquely from upper sides facing each other, and positions at which the reflecting light beams are incident on respective predetermined detection surfaces are detected by position detectors (102 and 104) for outputting respectively as position detection signals. The displacement of the surface to be measured is detected by obtaining the difference between or the sum of respective position detection signals so as to cancel the components in the directions opposite to each other out of the components indicating changes from the reference position included in respective position detection signals. <IMAGE>