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.