PROBLEM TO BE SOLVED: To realize highly accurate surface extraction that is robust against CT value fluctuations. SOLUTION: A voxel having a constant CT value is extracted from volume data acquired by X-ray CT to generate an isosurface mesh M, and at each vertex p of the mesh M, a gradient vector g of the CT value of the vertex p is calculated. A plurality of sample points S are calculated in the positive and negative directions of the calculated gradient vector g, the gradient norm N of the CT value at each generated sample point S is calculated, and the sample having the maximum calculated gradient norm Nm The vertex p of the isosurface is modified by moving the vertex p to the point Sm. [Selection diagram] Figure 4
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