A medium transport device includes a first roller pair, a second roller pair disposed downstream of the first roller pair, and an ultrasonic detection section disposed between the first roller pair and the second roller pair. The ultrasonic detection section includes a transmission sensor chip, a receiving sensor chip, a transmission board on which the transmission sensor chip is provided, and a receiving board on which the receiving sensor chip is provided. A first axis is inclined with respect to a surface of the medium passing between the transmission sensor chip and the receiving sensor chip and at least a part of the transmission sensor chip and at least a part of the receiving sensor chip are within a range of the second roller pair in a normal direction with respect to a surface of the medium at a nip position of the second roller pair.