Transmitted X-ray data is acquired by irradiating X-rays of appropriate doses determined for the portions of a section containing the major axis and the portions thereof containing the minor axis respectively. An X-ray irradiating/detecting device consists mainly of an X-ray irradiator that includes an X-ray tube (20) and irradiates a fan-shaped X-ray beam (400), and an X-ray detector (24) that has a plurality of X-ray detecting elements arrayed in a direction in which the fan-shaped X-ray beam spreads and that is opposed to the X-ray irradiator with an object of imaging (8) between them. The X-ray irradiating/detecting device is rotated (2,4) about the object in order to acquire transmission X-ray data stemming from a plurality of views. At this time, the dose of the X-ray beam is differentiated between predetermined angular ranges of a section of the object shaped like an oval, which extend with the minor axis of the oval section as a centerline, and the other angular ranges.
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