1. An image analyzing apparatus for extracting voids from a three-dimensional image of a fiber-reinforced composite material, the image analyzing apparatus comprising: a processor which executes image processing to the three-dimensional image, wherein the processor: binarizes the three-dimensional image and creates a binary image; transforms the binary image into a distance and creates a distance image; executes closing processing to the binary image by using the distance image; extracts voids from differences between images before and after the closing processing; among the extracted voids, classifies voids that are adjacent to a background voxel as open voids, and classifies voids that are not adjacent to a background voxel as closed voids; executes opening processing to the open voids in order to eliminate fake voids; and in the closing processing: executes expansion processing to the binary image by dividing the binary image into a plurality of two-dimensional images, expanding each of the plurality of two-dimensional images by using the distance image corresponding to each of the plurality of two-dimensional images, and calculating a sum set of the plurality of two-dimensional images that were expanded, and executes contraction processing to the binary image that underwent the expansion processing by dividing the binary image that underwent the expansion processing into a plurality of two-dimensional images, contracting each of the plurality of two-dimensional images by using the distance image corresponding to each of the plurality of two-dimensional images that were divided, and calculating a sum set of the plurality of two-dimensional images that were contracted.