A medium transport device includes a feeding roller which sends a medium to a downstream side; and a detecting unit which can detect the medium sent by the feeding roller at a predetermined position on a transport path, in which the detecting unit includes a lever member of which a posture is changed when being in contact with the medium, and a detecting portion which detects a change in posture of the lever member, and the lever member applies an external force which causes at least a part of the medium to be bent in a gravity direction by being in contact with the medium in a predetermined posture.