A validating machine 30 according to the present invention is provided with a validation sensor 2 having a first-side light emitting device 8 and a first-side light receiving device 10 disposed closely to each other and a validation sensor 2′ having a second-side light emitting device 8′ and a second-side light receiving device 10′ disposed closely to each other so that the validation sensor 2 and the validation sensor 2′ are disposed opposite to each other on a first side and on a second side of a bill 4. The first-side light emitting device 8 and the second-side light emitting device 8′ are controlled so as to emit light at their respective emission timings different from each other. The validating machine 30 performs composite detection to make the first-side light receiving device 10 detect first-side reflected light La1 emitted from the first-side light emitting device 8 and reflected on the first side of the bill 4 and to make the second-side light receiving device 10′ detect transmitted light La2 transmitted by the bill 4 and second-side reflected light Lb emitted from the second-side light emitting device 8′ and reflected on the second side of the bill 4, so as to validate compositions of the both sides of the bill 4.