PURPOSE: To inspect a curved article with high sensitivity and accuracy by scanning the surface of the article with a sensor head having chips and a probe which are brought into contact with the surface along a prescribed path.

CONSTITUTION: When the two contact chips 12 of a sensor head 10 stop on the surface 11 of a segment at a distance S, the chips 12 draw a chord 14 having the length S of a circle of R in radius. A third contact point has the freedom of moving in the direction perpendicular to the surface of an article to be measured and can be given by means of a probe 15 attached to the head 10. When the probe 15 is on the straight line between the chips 12, an arbitrary recessing and projecting section of the surface 11 displaces the probe chip to the upper or lower side of the chord 14 by a distance (h). When the probe 15 is at the exact center between the chips 12, a rectangular equilateral triangle having sides 1/2 S, R-h, and R is formed. Since the length S is known, the decision of the radius R only depends on the value of the distance (h) continuously given by the output of the probe 15.