To find the weight of a supporting point, which is not positioned in an orthogonal coordinate raster, by finding the weight for the supporting point from the points of a weight cube, converting the function values of points through evaluation functions to function values, multiplying the function values with correspondent interpolation weight and adding the results.

An X-ray radiator 1 and a detector 2 are rotated with a system axial line 4 as a center, and a reagent 3 is irradiated with fan-shaped X-ray beams 5 radiated from the X-ray radiator 1 to different directions. Data supplied from the detecting elements of the detector 2 are supplied to a calculation unit 6 and the calculation unit 6 reconstitutes the tomographic image in the irradiated volume of the reagent 3 and reproduces it on a monitor 7. In this case, the weight for the supporting point is converted to the function values at the points of cube positioned in the three-dimensional interpolation window of 3D griding process at the time of weight interpolation and multiplied with the mutually correspondent interpolation weight, and the results are added.