In an enclosure inspection apparatus 11, when a sealed letter 2 is inputted into a sealed letter loading station A, visual inspecting means 27 and X-ray inspecting means 33 determine thickness of the sealed letter and whether an enclosure in the sealed letter is a predetermined suspected object. The sealed letter not less than a predetermined thickness is rejected in first sorting station D, and the sealed letter which is not thicker than the predetermined thickness and in which the suspected object is not detected is conveyed as it is to second sorting station G. As for the sealed letter which is not thicker than the predetermined thickness and in which the suspected object is detected, after the suspected object in the sealed letter is positioned in a positioning station E, a suspected object inspecting station F determines whether the above-described suspected object is a predetermined object, such as an explosive or a narcotic drug, using a terahertz wave.It is possible to detect promptly the presence of the objects even if there are a large number of sealed letters.