1. A vortex extraction apparatus comprising: a data acquisition section that acquires flow velocity data at a plurality of points distributed in a fluid space; a first numerical data calculation section that calculates, for each vertex of each of voxels into which the inside of the fluid space is split, first numerical data indicating whether the vertex is located inside or outside a vortex region in the fluid, based on the flow velocity data acquired by the data acquisition section; a text data creation section that determines a center line of the vortex in the fluid based on the flow velocity data acquired by the data acquisition section and determines the size of the vortex region around the center line based on the first numerical data on each vertex calculated by the first numerical data calculation section, the text data creation section creating text data indicative of the center line and the size; a polygon count reduction processing section that counts the number of polygons defining a surface of the vortex region and each including an isosurface of the first numerical data calculated by the first data calculation section and integrates the plurality of voxels with new voxels into which the fluid space is roughly split until the number of polygons is equal to or smaller than a set value, to decimate the first numerical data and thereby obtain first numerical data corresponding to each vertex of each of the new voxels; and a data output section that outputs the text data created by the text data creation section and the first numerical data obtained by the polygon count reduction processing section and corresponding to each vertex of each of the new voxels.