PURPOSE: To prevent the lowering of reading ratio when the medium mark printed on copying paper is read and even when the density of the medium mark is low.

CONSTITUTION: This device has a detector 1 mounted on a carriage, a means making the detector 1 detect a medium mark by moving the detector 1 to a reading termination location, a means making the detector 1 detect the medium mark by moving the detector 1 to a reading starting location and an analog- digital conversion part 2 converting the analog signal of the detector 1 into a digital signal. The analog-digital conversion part 2 is provided with a comparator comparing second input voltage formed by delaying first input voltage and first input voltage. Even if the influence of a transparent passing is received when the medium mark is made to be detected by moving the detector 1 to the reading termination location, the influence of the transparent passing is not received when the medium mark is made to be detected by moving the detector 1 to the reading starting location. Therefore, the lowering of reading ratio is prevented.