PURPOSE: To maintain stable performance by means of a simple structure, at a low cost with high precision, for a long time by arranging a roof prism in a position opposed to a projected light receiving unit so as to pass a paper sheet through between the projected light receiving unit and the roof prism. ;CONSTITUTION: Projected light receiving units 45, 46 and a roof prism 50 are arranged so that a paper sheet 2 is passed through between them, and a roof prism face in the rear part of a reflection face of the roof prism 50 is surrounded by means of a cover 51 via an air layer 55. The roof prism 50 is molded by using a material such as acrylic resin and polycarbonate resin. When these materials are used, the reflection faces 52, 53 of the prism 50 serve as total reflection faces, so that a high reflectance can be provided without forming any reflection film. Therefore, the prism 50 is resistant against corrosion and inexpensive. In the roof prism 50, an illuminating angle of the reflected light to the light receiving unit is not changed even when an arrangement angle is shifted a little, so that influence due to vibration during use is inconsiderable, and stable performance can be provided for a long time.;COPYRIGHT: (C)1996,JPO
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