An isosurface mesh M is generated by extracting voxels having a certain CT value from volume data obtained by X-ray CT. A gradient vector g of a CT value is calculated at each vertex p of the isosurface mesh M. A plurality of sample points S are generated in positive and negative directions of the calculated gradient vector g. Gradient norms N of CT values at the respective generated sample points S are calculated. The vertex p of the isosurface mesh is moved and corrected to a sample point Sm having the maximum norm Nm calculated.