1. An analyzer of bone mineral density using CT image data of a phantom having a known bone mineral density, the analyzer comprising: a known data storage part that stores known data of bone mineral density for a phantom; a histogram production part that produces a histogram of region number relative to a CT value for three-dimensional CT image data of the phantom; a correspondence determination part that determines correspondence between a CT value and a bone mineral density by correlating CT values showing respective peaks of the produced histogram with the known data of the phantom; and an analysis part that decides a bone mineral density for three-dimensional CT image data of a subject using the determined correspondence.