1. A tracer control method which calculates a tracing direction and a tracing velocity on the basis of displacement signals from a tracer head and controls tracing of a concavity of a model, comprising the steps of: (a) clamping the tip of the tracer head at a Z-axis position of a predetermined depth in the concavity of the model; (b) performing contour tracing along the entire circumference of the inner wall of the concavity of the model while holding the tip of the tracer head in the clamped state and, while doing so, storing the coordinates of maximum and minimum points where a predetermined one of the X and Y coordinates of the tip of the tracer head has its maximum and minimum values on the contour tracing path; (c) feeding the tracer head to a point inside one of the maximum and minimum points while maintaining it in the clamped state after completion of the contour tracing; (d) effecting X-Y plane tracing relative to the inner wall of the concavity of the model from the abovesaid point to the other one of the minimum or maximum points of the concavity while retaining the tip of the tracer head in the clamped state; and (e) further lowering the tip of the tracer head along the inner wall of the concavity of the model from the end position of the X-Y plane tracing and clamping it at a new Z-axis position of a predetermined deeper depth; wherein the steps (b) to (e) are repeated in the newly clamped state until the tip of the tracer head reaches a further predetermined depth.