A computed tomographic apparatus comprising: an X-ray scanner for: dividing X-rays penetrating a subject by energy bands; capturing scout images of the respective energy bands; and wherein the X-ray scanner comprises an X-ray source for irradiating X-rays to the subject and configured to capture the scout images by irradiating the X-rays to the subject moving in a direction parallel to a rotating shaft of a gantry and perform CT scanning by rotating the gantry and irradiating X-rays to a scanning region; an X-ray detector; and the gantry on which the X-ray source and the X-ray detector are arranged to face each other, wherein the X-ray detector is configured for detecting X-rays penetrating the subject, and wherein the X-ray detector comprises a photon counting detector configured to set multiple threshold energy levels and count photons greater than the respective threshold energy levels for dividing the detected X-rays by the energy bands, wherein the CT apparatus further comprises: an image processor for generating substance images of substances of the subject, based on the scout images; a display for displaying a substance image of the substance images; and an input unit for receiving the scanning region for the displayed substance image, CHARACTERIZED IN THAT the display is configured to display the substance image and a scout image of the scout images, the displayed substance image and the displayed scout image overlaying each other, and BY a user operable controller (c) for user manipulation to vary respective opaqueness of the substance image and the scout image to emphasize either the substance image or the scout image and help the user to easily establish a scanning region for input through the input unit. an X-ray scanner for: dividing X-rays penetrating a subject by energy bands; capturing scout images of the respective energy bands; and dividing X-rays penetrating a subject by energy bands; capturing scout images of the respective energy bands; and wherein the X-ray scanner comprises an X-ray source for irradiating X-rays to the subject and configured to capture the scout images by irradiating the X-rays to the subject moving in a direction parallel to a rotating shaft of a gantry and perform CT scanning by rotating the gantry and irradiating X-rays to a scanning region; an X-ray detector; and the gantry on which the X-ray source and the X-ray detector are arranged to face each other, an X-ray source for irradiating X-rays to the subject and configured to capture the scout images by irradiating the X-rays to the subject moving in a direction parallel to a rotating shaft of a gantry and perform CT scanning by rotating the gantry and irradiating X-rays to a scanning region; an X-ray detector; and the gantry on which the X-ray source and the X-ray detector are arranged to face each other, wherein the X-ray detector is configured for detecting X-rays penetrating the subject, and wherein the X-ray detector comprises a photon counting detector configured to set multiple threshold energy levels and count photons greater than the respective threshold energy levels for dividing the detected X-rays by the energy bands, wherein the CT apparatus further comprises: an image processor for generating substance images of substances of the subject, based on the scout images; a display for displaying a substance image of the substance images; and an input unit for receiving the scanning region for the displayed substance image, CHARACTERIZED IN THAT the display is configured to display the substance image and a scout image of the scout images, the displayed substance image and the displayed scout image overlaying each other, and BY a user operable controller (c) for user manipulation to vary respective opaqueness of the substance image and the scout image to emphasize either the substance image or the scout image and help the user to easily establish a scanning region for input through the input unit. an image processor for generating substance images of substances of the subject, based on the scout images; a display for displaying a substance image of the substance images; and an input unit for receiving the scanning region for the displayed substance image, CHARACTERIZED IN THAT the display is configured to display the substance image and a scout image of the scout images, the displayed substance image and the displayed scout image overlaying each other, and BY a user operable controller (c) for user manipulation to vary respective opaqueness of the substance image and the scout image to emphasize either the substance image or the scout image and help the user to easily establish a scanning region for input through the input unit.