1. An X-ray CT device comprising an X-ray generation part, an X-ray detection part oppositely disposed to the X-ray generation part and having a plurality of X-ray detection elements, a correction part for correcting data detected by the X-ray detection part, and an image reconstruction part for reconstructing a CT image by using corrected data, wherein: the correction part comprises a beam hardening correction part for correcting the detected data on the basis of a BH correction function representing relation between target values of X-ray absorption characteristic of a predetermined subject and X-ray absorption characteristic of the predetermined subject under the influence of beam hardening, and the beam hardening correction part comprises an X-ray absorption characteristic calculation part for calculating X-ray absorption characteristic by using errors between simulation-calculated values of X-ray absorption characteristic obtained beforehand for a virtual phantom by simulation and actually measured values of X-ray absorption characteristic of a phantom equivalent to the virtual phantom, and a BH correction function calculation part for calculating the BH correction function by using the X-ray absorption characteristic calculated by the X-ray absorption characteristic calculation part and target values of X-ray absorption characteristic calculated beforehand, and corrects data measured for a test subject on the basis of the BH correction function calculated by the BH correction function calculation part.