An X-ray that has passed through a target object 13 is detected by a two-dimensional X-ray detector 14 capable of energy discrimination, to determine an actual X-ray strength at a specific energy level. The target object 13 is rotationally scanned in predetermined angular steps around an axis perpendicular to the X-ray. The target object 13 is imaginarily divided into a large number of micro-sized unitary cubic cells, and each detection element 14a is considered to be receiving an X-ray that has passed through a plurality of unitary cubic cells. A data processor 6 creates a system of equations including a measured intensity ratio between the transmitted and direct X-rays and a theoretical X-ray intensity ratio calculated from the mass absorption coefficients, unknown weight ratios and density of the elements contained in each unitary cubic cell. The number of equations is equal to or larger than the total number of the elements having unknown weight ratios and the density. In this manner, one can measure the three-dimensional distribution of the weight ratio of each element and/or compound and the density in the target object containing a plurality of known kinds of elements and/or compounds.