1. A color gamut cross-section acquiring method for acquiring a cross-section of a color gamut of an image forming apparatus taken on a specific plane, the method comprising: acquiring color gamut boundary line segments that are line segments for defining a boundary of the color gamut; acquiring cross-section boundary line segments that are line segments for defining a boundary of the cross-section; and acquiring, as endpoints of the cross-section boundary line segments, intersection points between the color gamut boundary line segments and the specific plane, wherein the acquiring cross-section boundary line segments includes: acquiring a shortest possible route passing through each of the intersection points acquired as the endpoints exactly once; acquiring each of line segments making up the route as a provisional cross-section boundary line segment; acquiring a set of verification line segments by connecting intersection points between the color gamut boundary line segments and a plane intersecting with the provisional cross-section boundary line segment at a point other than the endpoints of the provisional cross-section boundary line segment; and acquiring the provisional cross-section boundary line segment as a cross-section boundary line segment if the provisional cross-section boundary line segment intersects with the set of verification line segments.