1. An X-ray computed tomography (CT) apparatus, comprising: an X-ray irradiation unit for irradiating X-rays; an X-ray detecting unit for detecting the X-rays that have passed through a subject and generating X-ray detecting data; a calculation unit of projection data for subjecting the X-ray detecting data to logarithmic conversion and generating projection data; a filter processing unit for smoothing at least one of the X-ray detecting data and the projection data; and an image calculation unit for calculating from data acquired in the filter processing unit, a CT image which represents a distribution of X-ray attenuation coefficient, wherein the filter processing unit generates boundary data by using at least a part of the X-ray detecting data and the projection data, and performs smoothing processing on at least one of the X-ray detecting data and the projection data, using the boundary data as a threshold, and wherein the filter processing unit compares the X-ray detecting data with the boundary data, or the projection data with the boundary data, and sets a smoothing degree of smoothing processing according to a difference between the compared data.