A trace guide (TG) that has been set within a manual trace reference cross-section (58) is illustrated with a dashed line. The trace guide (TG) is obtained from three-dimensional contour information based on already-completed manual tracing of a first sheet. Therefore, the user draws a trace line (TL) corresponding to the contour of target tissue on a second sheet of the manual trace reference cross-section (58) while referring to the trace guide (TG) and also checking a tomographic image of the target tissue within the manual trace reference cross-section (58). The user may: draw the trace line (TL) in full; use apart of the trace guide (TG) without alteration as the trace line (TL) and correct the remaining part to serve as the trace line (TL); or use the trace guide (TG) without alteration as the trace line (TL).