This invention provides an X-ray CT image processing method that allows more flexible expression by expressing X-ray absorption coefficients probabilistically, makes it possible to acquire reconstructed images that are comparable to those obtained by conventional methods but involve lower X-ray doses, and can reduce beam-hardening artifacts. A probability distribution for the observation of projected X-rays is set and statistical inference is performed. Said probability distribution is expressed in terms of the process of observing a multiple-X-ray sum resulting from multiple projected X-rays being incident upon a detector. Bayesian inference in which the expected value of the posterior distribution is used for statistical inference is performed on the basis of a prior distribution for X-ray absorption coefficients, said prior distribution having parameters for the material and the observation process in terms of which the multiple-X-ray sum is expressed.
