PURPOSE: To enable indexing in parallel with a model surface without any effect of friction thereupon and ensure high profiling speed by providing a means for correcting an indexing error in a profiling direction accruing from friction with the model.

CONSTITUTION: The position on a tracer head 3 is detected in such a way as P1(x1, y1, z1) and P2(x2, y2, z2) from a position detector 4, and a position orthogonal with a locus profiled at each point, namely E1 for the locus P0-P1 and E2 for P1-P2 are obtained. By using the data so obtained, vector operation En-n is done for the displacement of n of the tracer head 3 at each point and a correction value c is obtained for an error displacement. Then, when vector reduction is made from a tracer displacement signal , displacement due to effect by friction is canceled and only displacement N perpendicular to a model surface 2 can be taken out. When a profiling direction is indexed, therefore, only the displacement N perpendicular to the model surface 2 is taken out, thereby enabling indexing in parallel with the model surface 2 and the speed-up of profiling.