PURPOSE:To ensure the detection of the knife mark, by detecting the reflected light of the light, which is projected from the front and back in the conveying direction of a single slice plate by a light receiver, and performing the binary coding of the signal outputted from the light receiver in synchronization with switching operation of light projectors. CONSTITUTION:Belt shaped projected light is inputted to the single slice plate 1 from the projector 2. Another light projector 2A is arranged at the plane symmetrical position with respect to projector 2, with a plane which is perpendicular to an incident part as a center. The light is irradiated from the front and back with respect to the light receiver 3 in the conveying direction of the single slice plate 1. The light is alternately projected to each single slice plate 1 which is sequentially sent continuously from both projectors 2 and 2A. The reflected light is received and detected by the light receiver 3, and the knife mark X is detected. The output of the light receiver 3 is inputted to a binary coding circuit 9 through a bypass filter 8. The data with respect to the presence or absence of the knife mark X is obtained from the output of the circuit 9.
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