PURPOSE: To reduce the cost while enhancing the accuracy of measurement by feeding a signal from an encoder and a light receiving unit for detecting a reflected light beam to an evaluation unit for calculating the linear dimensions of a structure.

CONSTITUTION: A wafer 12 is arranged on a table 11 and positioned in x, y directions by means of a screen analyzer 28 and the angular position thereof is determined. During the operation, wafer surface is introduced along the focal plane of an objective lens 4 by means of an autofocus unit 27. A computer 20 in an evaluation unit 62 separates the z direction driver for table 11 from a motor controller 26 and turns flat plates 30a, 30b thus guiding a measuring spot focused onto the wafer surface above a structure to be measured. During the cycle, position of the focal plane is varied by a piezoelectric driver 18 for adjusting the focusing motion of lens 4 finely. Maximum intensity of a measuring light beam reflected on the wafer 12 and passed through a photomultiplier 15 is correlated by the computer 20, with values related to the x, y positions in the plane of measuring spot and the value z of height obtained from a measuring system.