PURPOSE: To easily and correctly detect displacement even when the center point of a stylus is displaced from the zero point position in the stylus displacement detecting circuit of a tracer head on a tracer control device or the like.

CONSTITUTION: When a power source is turned on and the displacement of a stylus 5 is inputted, a displacement quantity signal SS is outputted from a relative displacement detecting circuit 42 in response to the displacement of the stylus 5. When the stylus 5 is first returned to the zero point after the displacement is inputted, a grid signal GS is outputted from a zero point return detecting circuit 41, and the control signal PS of a flip-flop circuit 43 rises. When the control signal PS rises, a gate circuit 44 outputs a count signal CTS in response to the displacement quantity signal SS. The displacement quantity in the X-axis direction of the stylus 5 is fed to a digitizing control device.