PURPOSE: To reliably detect a duplicate feed even for various sheets with different light permeability factors by detecting a duplicate feed of the sheets by means of the permeating light of the sheets and making the detection sensitivity selectable in a device transporting the sheets.

CONSTITUTION: Light rays are illuminated from a light emitter 2 to a sheet 1 transported by a roller, etc., its permeating light is detected by a photosensor 3, amplified by an amplifier 6, then compared with a reference level V0 for detecting a duplicate feed from a reference level generator 7 at a comparator circuit 8 to generate a detection signal 9. The emitted light quantity of the light emitter 2 is adjusted by selecting either of power supplies 4-1W4-n with a selection switch, for example, to change the power supply to be fed. Thereby, the emitted light quantity of the light emitter 2 is changed according to different light permiability factors depending on the quality and thickness, etc. of the sheets 1, thus the light emitter 2 emits light rays at a luminous level not causing the miss detection of a duplicate feed. Therefore, a duplicate feed can be reliably detected.