1. A method of visualizing structured grid data on a monitor using computer graphics technology, the structured grid data including first and second structured grid data respectively arranged for first and second component structured grids of a spherical structured grid that is constituted by complementarily combining said first component structured grid and said second component structured grid to form a sphere, said first component structured grid being constituted by extracting from a latitude-longitude grid in a first spherical coordinate system a range between a certain value of −45 degrees or less and a certain value of 45 degrees or more, excluding −90 and 90 degrees, in latitude and a range between a certain value of 45 degrees or less and a certain value of 315 degrees or more, excluding 0 and 360 degrees, in longitude, said first spherical coordinate system using a z axis of an xyz space as a central axis, said second component structured grid being constituted by extracting from a latitude-longitude grid in a second spherical coordinate system a range between a certain value of −45 degrees or less and a certain value of 45 degrees or more, excluding −90 and 90 degrees, in latitude and a range between a certain value of 45 degrees or less and a certain value of 315 degrees or more, excluding 0 and 360 degrees, in longitude, said second spherical coordinate system using a y axis of the xyz space as a central axis, the method comprising the steps of: creating a first graphic object using said computer graphics technology after setting said first component structured grid in a first local xyz coordinate system used in computer graphics and disposing the first structured grid data on said first component structured grid; creating a second graphic object using said computer graphics technology after setting said second component structured grid in a second local xyz coordinate system used in said computer graphics and disposing the second structured grid data on said second component structured grid; rotating the second graphic object by 90 degrees with respect to an x axis of said first local xyz coordinate system, together with said second local coordinate system, and then rotating the resulting second graphic object by 180 degrees with respect to a z axis of said first local xyz coordinate system; visualizing a graphic object, which has been obtained by combining the first graphic object and the rotated second graphic object, on said monitor as an image, using said computer graphics technology; and, determining an overlapping region where said first component structured grid overlaps with said rotated second component structured grid, and selecting one or the other of the structured grid data disposed on said first component structured grid and the structured grid data disposed on said second component structured grid, which are to be visualized in the overlapping region, the determination step being implemented in the step of visualizing said graphic object on said monitor, wherein the determining step implements a region determination criterion defined to visualize the selected structured grid data without polygon creation and normal computation on a polygon vertex.