An X-ray computed tomography apparatus includes an X-ray tube, an X-ray detector, a volume data reconstruction unit, a lung field region specifying unit, a discrimination unit, an image generation unit, and a display unit. The volume data reconstruction unit reconstructs first volume data of a chest region of an object based on an output from the X-ray detector. The lung field region specifying unit specifies a lung field region of the object in the first volume data. The discrimination unit generates second volume data in which a low CT value region is discriminated from a region other than the low CT value region in the lung field region. The image generation unit generates an image representing a two-dimensional distribution concerning an existing ratio of the low CT value region to the lung field region based on the second volume data.
