PURPOSE: To simply perform tracer control without requiring zero point adjustment when a power source is turned on.

CONSTITUTION: An X-axis scale 41 and a Z-axis scale 42 are provided on a tracer head 10, and an X-axis differential transformer 43 and a Y-axis differential transformer 44 are provided. When the power source of a control device is again turned on after it is turned off, for example, positions of cores of the differential transformers 43, 44 are drifted by the residual displacement quantity, thus the absolute position of a stylus 20 can be detected when detected data of the differential transformers 43, 44 are used. The dynamic displacement of the stylus 20 thereafter can be known from the detected data of the scales 41, 42. No zero point adjustment is required when the power source is turned on.