1. An X-ray CT apparatus comprising: an X-ray radiation unit configured to apply a first X-ray having a first energy spectrum and a second X-ray having a second energy spectrum different from the first energy spectrum to a subject; an X-ray data acquisition unit configured to acquire first X-ray projection data of the subject based on the first X-ray applied by said X-ray radiation unit and second X-ray projection data of the subject based on the second X-ray applied by said X-ray radiation unit; and an image-reconstruction device configured to determine difference information between the first X-ray and the second X-ray with respect to an image of the subject based on the first X-ray projection data and the second X-ray projection data, segment a pixel area of the image into at least two pixel areas based on the difference information, and set at least a portion of the segmented pixel areas to a dual energy image obtained by a weighted subtraction process based on the first X-ray projection data the second X-ray projection data having different energy spectrums.