A visualization program that renders an internal organ and causes a computer to execute: creating a three-dimensional model (M) of the internal organ by setting a set of tetra elements (Ej(ti)), respective tetra elements being tetrahedral unstructured grid data, wherein first tetra elements forming an external surface or an internal surface of the model (M), the internal surface being farther from a viewpoint than said external surface, have a normal vector directed outside the model, and tetra elements exclusive of the first tetra elements have no normal vector; setting (802;1202) a designated area at the external surface of the three-dimensional model (M) of the internal organ; generating (804, 805; 1204, 1205) a volumetric shape, along a plurality of normal vectors originating from the designated area, by pushing out the designated area as an extrusion face a particular distance in a posterior direction towards the internal surface of the three-dimensional model (M) of the internal organ; while pushing out the designated area, identifying first tetra elements of the internal surface included in the extrusion face, from an angle θ formed by a normal vector and the direction of a sight vector, having the normal vector in said posterior direction, wherein said pushing out of the designated area is until the first tetra elements having the normal vector in the posterior direction are no longer detected; identifying (806; 1206) a tetra element group that overlaps the volumetric shape and a shape of the internal organ; changing (807; 1207) a degree of transparency of the identified tetra element group; and rendering (808;1208) the shape of the internal organ to include the identified tetra element group for which the degree of transparency has been changed. creating a three-dimensional model (M) of the internal organ by setting a set of tetra elements (Ej(ti)), respective tetra elements being tetrahedral unstructured grid data, wherein first tetra elements forming an external surface or an internal surface of the model (M), the internal surface being farther from a viewpoint than said external surface, have a normal vector directed outside the model, and tetra elements exclusive of the first tetra elements have no normal vector; setting (802;1202) a designated area at the external surface of the three-dimensional model (M) of the internal organ; generating (804, 805; 1204, 1205) a volumetric shape, along a plurality of normal vectors originating from the designated area, by pushing out the designated area as an extrusion face a particular distance in a posterior direction towards the internal surface of the three-dimensional model (M) of the internal organ; while pushing out the designated area, identifying first tetra elements of the internal surface included in the extrusion face, from an angle θ formed by a normal vector and the direction of a sight vector, having the normal vector in said posterior direction, wherein said pushing out of the designated area is until the first tetra elements having the normal vector in the posterior direction are no longer detected; identifying (806; 1206) a tetra element group that overlaps the volumetric shape and a shape of the internal organ; changing (807; 1207) a degree of transparency of the identified tetra element group; and rendering (808;1208) the shape of the internal organ to include the identified tetra element group for which the degree of transparency has been changed.