PURPOSE: To perform the digitizing of high accuracy not restraint by a feeding speed by obtaining the coordinate value of each fine and rectangular apex point on a model face based on the measurement values of the sampling times of the previous and this times fed from two distance detectors, finding the slope line vector based on the coordinate values of the necessary three apex points therein and obtaining the slope line direction data.

CONSTITUTION: The 1st measurement value of a 1st noncontact distance detector 30a subjected to sampling at the sampling time of the previous time and the 2nd measurement value of a 2nd distance detector 30b are stored by a memory means 13. The slope line direction data of the model surface is found by an arithmetic means 11 based on the at least three measurement values of the 1st and 2nd measurement values, the 3rd measurement value of the 1st non- contact distance detector 30a subjected to sampling at the sampling time of this time and the 4th measurement value of the 2nd non-contact distance detector 30b. Consequently the need for making the feeding direction of a tracer head 3 in zigzag is eliminated and the digitizing of a high accuracy not constraint by the feeding speed can be performed.