1. An X-ray computer tomography apparatus, comprising: an X-ray source configured to irradiate X-rays onto an object to be examined; an X-ray detection unit configured to detect X-rays transmitted through the object; a driving unit configured to rotate at least one of the X-ray source and the X-ray detection unit around the object; an image data generation unit configured to generate image data on the basis of projection data using the X-ray detection unit; a density calculating unit configured to calculate contrast medium density in the region of interest being set in the image data from the image data generation unit after injecting contrast medium into the object; an irradiation condition setting unit configured to set a first irradiation condition for first scanning under which low-dose X-rays are irradiated, and a second irradiation condition for second scanning under which high-dose X-rays are irradiated on the basis of the contrast medium density; and a blood-flow information calculating unit configured to calculate blood-flow information on the basis of the image data obtained by the first scanning and the second scanning.