1. A method for quantitatively determining bone mineral mass by a CT system, comprising scanning an objective region together with plural samples produced by mixing a water equivalent material with various ratios of a standard material equivalent to bone mineral mass and determining the bone mineral density of the objective region with reference to the CT numbers of the plural samples with various densities of the standard material equivalent to bone mineral mass, wherein corrected CT number of each of the samples with various densities of the standard material equivalent to bone mineral mass is calculated by substituting the CT number derived from the water equivalent material and contained in the CT number of each of the samples with the CT number of blood or a standard material equivalent to blood in the cross sectional image data generated from such scanning, whereby the bone mineral density of the objective region is determined on the basis of the corrected CT number of each of the samples.