An X-ray CT apparatus comprising a storage device which stores therein a first table where fat contents are respectively associated with the values of the ratios of CT numbers, a CT number measurement unit which irradiates an object with X-rays under a plurality of different irradiation conditions, and which obtains actually-measured CT numbers for the respective irradiation conditions, and a fat content extraction unit which calculates the ratio of the actually-measured CT numbers for the respective irradiation conditions, and which extracts the fat content corresponding to the ratio of the actually-measured CT numbers, by referring to the first table.