1. An X-ray CT system comprising, an X-ray irradiation unit for irradiating X-rays having multiple types of different energy spectra, an X-ray detection unit for detecting the X-rays after passing through an object, an image calculation unit for calculating an image of X-ray attenuation coefficients respectively for the multiple types of X-rays based on each detection signal from the X-ray detection unit, a separation calculator for applying to a map for separation which indicates a relationship between multiple types of X-ray attenuation coefficients and a composition of the object, values of the X-ray attenuation coefficients in association with a corresponding area in the image of multiple types of X-ray attenuation coefficients, so that the composition of the area is separated, a map formation unit for separation for forming the map for separation, and an input unit for accepting from an operator, an input regarding an X-ray irradiation condition of the X-ray irradiation unit and multiple compositions to be separated, wherein, the map formation unit for separation performs arithmetic processing to obtain existing probabilities of the multiple compositions under the X-ray irradiation condition being inputted in the input unit, and forms the map for separation based on the existing probabilities.