To provide an image generator capable of reconstructing a CT image suppressing the degradation of space resolution while enjoying the profit such as artifact reduction by carrying out fan/parallel conversion.

The image generator includes: a converting unit 603 converting fan beam projecting data to unequal distance parallel beam projecting data in which X-ray beams are spaced unequally by applying converting processing doing the sorting of the projecting data of respective channels but not doing the interpolating processing equalizing the distances of the X-ray beams corresponding to the projecting data of the respective channels to the fan beam projecting data; and a reconstructing unit 602 specifying the positions of the X-ray beams corresponding to the projecting data of the respective channels of the unequal distance parallel beam projecting data using a trigonometric function having a parameter of an opening angle α from the center line of the fan beam X-ray to the X-ray beam when the projecting data are collected, for example sin (α), and doing image reconstructing processing using specified position information.