An X-ray CT device and a computing unit to use a projection-based method for image reconstruction are included. The computing unit sets a coordinate space having coordinate axes of the thicknesses of base materials and likelihood of said thicknesses, and then based on X-ray attenuation responses, executes: a first search to search in a direction perpendicular to a ridge direction of likelihood contours for a first estimated thickness having the highest likelihood, starting with an estimated thickness input value set in the coordinate space； a second search to search for a second estimated thickness having the highest likelihood, starting with a shifted starting point at a position shifted from the estimated thickness input value； and a third search to search on a line connecting the first estimated thickness with the second estimated thickness for the highest likelihood estimator having the highest likelihood, to obtain an estimated thickness output value.
