In one forward or backward trace path on a model, a jerk point in the surface configuration of the model is detected and the position of the jerk point is stored in a memory. The reduction of the trace velocity is started a predetermined distance short of the stored position of the jerk point in the trace direction and when a jerk point is detected again, the position of the jerk point previously stored in the memory is updated and the reduction of the trace velocity is released, thus achieving high accuracy trace.