A medium transport unit includes: a transport roller; a transport belt which transports a loaded medium by driving of the transport roller; a sensor array which includes a plurality of imaging elements arranged vertically to a transport direction of the medium and outputs reading signals which are detected and output by the plurality of imaging elements and indicate a density value of the medium on the transport belt; a comparator which outputs a digitalized signal of HIGH if the reading signal exceeds a threshold signal; and a signal output circuit which receives the digitalized signal and outputs a first value if the digitalized signal is transitioned from LOW to HIGH.