A CT scanner pre-scans a patient by radiating rays of a lower amount from one direction to obtain a perspective prescanning image. An operator defines a main scanning area, in which images for medical application are to be captured, with reference to the prescanning image displayed on a monitor screen. A site recognizer analyzes data of the prescanning image to recognize anatomical structures located in the defined main scanning area. Based on the recognized anatomical structures and order data designating a site of inspection, it is judged whether the main scanning area corresponds to the site of inspection designated by the order data. If not, a warning is displayed on the monitor screen. The site recognizer may recognize all anatomical structures contained in the prescanning image, so that a default main scanning area may be defined on the basis of the recognition results and the order data.