PURPOSE: To measure the surface profile of any object accurately regardless of the surface properties thereof by providing slits, corresponding in number to the identifiers, at each opening.

CONSTITUTION: A slit plate 4 is provided with different number of slits (s) at each opening E1-E4 for passing laser light through a lens and the number of slits represents the identifier at that opening. A laser light entering each opening is split by the number of slits thereat and then converted through an photoelectric converter into measurement pulse signal r1-r4. Each measurement pulse signal r1-r4 is inputted to a signal processing circuit at a timing shown on the figure. Number of slits (s) is pre-stored in the signal processing circuit for each opening E1-E4. When pre-stored number of pulse signals are not inputted, it is decided that the measurement failed at the measuring point corresponding to that number and a predetermined abnormality processing can be carried out.