An X-ray computed tomography apparatus includes a gantry, a reconstruction processing unit which generates first volume data on the basis of a projection data set which covers an angle range of 180.degree.+a fan angle and also generates second volume data on the basis of a projection data set which covers an angle range of 360.degree., a correction data generating unit which generates correction data for reducing a cone beam artifact on the basis of a difference between the first volume data and the second volume data, and a correction unit which corrects the first volume data on the basis of the correction data.