A control unit performs helical scanning an subject while moving a top along a direction substantially parallel to a body axis. An acquisition unit acquires projection data via an X-ray detector. A projection data extraction unit extracts a projection data set necessary for the reconstruction of image data associated with a predetermined slice position from the projection data. A weighting unit assigns a smaller weight to first projection data of the extracted projection data than a weight assigned to second projection data, the first projection data being acquired outside a predetermined period including a predetermined acquisition time of the projection data at the predetermined slice position, the second projection data being acquired within the predetermined period. A reconstruction processing unit reconstructs image data on the basis of the first projection data and the second projection data to which the weights are assigned.
