PURPOSE: To execute the accurate measurement by a method wherein a mark is put on an inflection point on a border line and a reflection light of a laser light is detected so that a position of the inflection point in the width direction is calculated and a position in the thickness direction specified by a position in the width direction is calculated.

CONSTITUTION: A tire tread T having a mark on an inflection point is placed on a tire member loading plate 9. A laser light from a He-Ne laser projector is cost on the top face of the tread T and the reflection light thereof is taken by a CCD camera 25 via a fourth mirror 24, then the image is integrally processed so that data of an approximate shape of a cross section of the tread T is obtained. The measurement is executed by means of upper and lower measuring devices 50, 51 in such a manner that optimum distances from the tread T are maintained. The CCD camera 25 in the measuring devices 50 specifies a position of the mark in the width direction. When the mark spans both of measuring windows, one of the measuring window having a greater analog addition value is selected so that a position of the mark in the X axis direction is specified and a position of the mark in the thickness direction is specified by an analog centroid method.