A pair of optical sensors are disposed above photopolymer plates and interleaf papers, which are alternately stacked with one another. These optical sensors basically have the same structure. However, the optical sensor is disposed so as to be inclined to an optical axis of reflected light, and therefore, a detection level of the optical sensor substantially becomes low. Accordingly, although the optical sensors each react to the reflected light from the photopolymer plate, the optical sensor does not react to the reflected light from the interleaf paper. As a result, it is determined whether the uppermost layer of the stack is the photopolymer plate or the interleaf paper. In a discriminating device of the present invention, general purpose optical sensors adapted to react to light having such a fixed intensity or greater, are used.