1. A threat detection system comprising: an image pickup device disposed so as to be able to pick up a scan projection image of an inspection subject from a first direction and a second direction; a computation unit for computing an X-ray absorption coefficient of an object contained in the inspection subject on the basis of a first scan projection image picked up from the first direction by the image pickup device and a second scan projection image picked up from the second direction by the image pickup device; a storage unit for retaining a database of X-ray absorption coefficients of threats; a decision unit for making a decision as to whether the computed X-ray absorption coefficient is included in the database; and a display unit for displaying the first scan projection image or the second scan projection image together with a result of the decision.