A medium transport device includes a feed roller pair, a stack portion that receives a medium transported by the feed roller pair between a support surface for supporting the medium in an inclined posture in which a downstream side in a transport direction is directed downward and an opposing surface opposing the support surface and stacks the medium, an alignment portion that aligns a downstream end of the medium stacked in the stack portion, and a control unit that controls a distance between the support surface and the opposing surface, in which the stack portion is configured to be capable of changing the distance and the control unit adjusts the distance according to a condition.