PURPOSE: To enable occurate detection of a distance between an object to be measured and a datum plane by measuring the space and position between plural slit images and correcting said distance even if the object to be measured is rugged.

CONSTITUTION: The light emitted from a light source 4 is irradiated through slits 5 (two slits) to an object 6 to be measured. The light reflected from the object irradiated with said light is detected by a two-dimensional CCD7. The CCD7 is driven by a CCD driving circuit 8 which generates a series signal 9. The signal 9 is converted to a binary signal 11 by a binary coding circuit and is inputted to a position coordinate circuit 12. A synchronizing signal 13 is inputted to the circuit 12 from the CCD driving circuit. The coordinates of the two slit images are given by the circuit 12. The circuit which calculates the distance (h) between a datum plane 2 and the object 6 from the coordinates of the tow slit images is an arithmetic circuit 14 for distance.