1. An X-ray CT apparatus comprising: an X-ray source that irradiates an object with X-rays; an X-ray detector that is disposed to oppose the X-ray source and detects X-rays having been transmitted through the object; a rotation board that is mounted with the X-ray source and the X-ray detector and is rotated around the object; an imaging control unit that performs imaging while modulating an irradiation X-ray dose on the basis of predetermined dose modulation data; a projection data generation unit that generates projection data on the basis of transmitted X-ray data detected by the X-ray detector; a storage unit that holds an image quality improvement table indicating image quality improvement effect amounts of a plurality of image quality improvement processes; a reference dose acquisition unit that sets a dose value corresponding to reference image quality as a reference dose; a reconstruction target dose acquisition unit that acquires an irradiation X-ray dose during imaging for image reconstruction target projection data, from the dose modulation data; an image quality improvement process selection unit that selects an image quality improvement process for obtaining the reference image quality from the image quality improvement table on the basis of a ratio between the reference dose and the irradiation X-ray dose acquired by the reconstruction target dose acquisition unit, and the image quality improvement effect amount; and a reconstruction processing unit that performs the image quality improvement process selected by the image quality improvement process selection unit on the image reconstruction target projection data, so as to reconstruct an image.