1. An optical coherence tomography apparatus for selectively visualizing and analyzing a vascular network in a choroidal layer comprising: an optical coherence tomography device, and a computer that obtains three-dimensional OCT tomographic images based on OCT-measured data acquired by the optical coherence tomography device and processes the three-dimensional OCT tomographic images, wherein the computer comprises: a means for acquiring image data of the choroidal vessels by selectively separating out only images of choroidal vessels from the three-dimensional OCT tomographic images, and a means for obtaining data to be used in a quantitative evaluation of a shape of the choroidal vessels based on the image data of the choroidal vessels; wherein the means for acquiring image data of the choroidal vessels performs extracting tomographic image data of the choroidal layer from the OCT-measured data, extracting data of image slices from the tomographic image data of the choroidal layer, which data of image slices are image data representing the choroidal layer sliced at equally pitched positions in a depth direction of the choroidal layer, and then acquiring the image data of the choroidal vessels from the data of image slices, and wherein the means for acquiring image data of the choroidal vessels is constituted in a manner that, for each of multiple windows of different sizes, each pixel in the image slice is binarized according to whether or not a pixel color density is equal to or higher than a pre-determined specified threshold in order to obtain an estimated vessel parts-extracted binary image, and in this estimated vessel parts-extracted binary image, those regions where a ratio of pixels with different binary data to each pixel in an applicable window is equal to or greater than a pre-determined specified value are deleted as pseudo vessels, while the estimated vessel parts whose diameter is smaller than a pre-determined specified diameter with respect to a dimension of the applicable window are also erased as noise and non-vessels, in order to obtain binary image data of the vascular network in the choroidal layer.