PURPOSE: To measure the digitizing data of a copying curved surface accurately and precisely without using an expensive computing device suitable to a high speed process, by realizing a computing process necessary for the feed of the copying control in a simple adding and subtracting computation.

CONSTITUTION: By finishing the movement of a tracer head 6 to an object position, the deflection between the actual position of a copying curved surface and the present position of the tracer head 6 is found from the displacement amount of a stylus in the detecting direction orthogonal to the measuring movement direction, and the actual position of the copying curved surface is stored as the digitalizing data of the copying curved surface in ROM 4. And the moving amount in the detecting direction orthogonal to the measuring movement direction when it is moved a specific amount in the measuring movement direction on a straight line combining two copying curved surface found immediate before the measuring is found. A deflection found beforehand is added to this moving amount so as to calculate the moving amount to the direction orthogonal to the measuring movement direction. A moving object position is found from the calculated moving amount and a specific moving amount in the measuring movement direction, and the tracer head 6 is moved to the object position.