A medium transport device includes a first transport route for discharging a medium processed by a processing unit, a second transport route for discharging the medium processed by the processing unit, and a medium receiving tray that is capable of switching a state between a medium receiving state of receiving the medium that is discharged through the first transport route and a medium unreceivable state achieved by performing state switching from the medium receiving state. The first transport route comes into a non-formed state and at least a part of the medium receiving tray forms the second transport route when the medium receiving tray is in the medium unreceivable state, and the second transport route comes into the non-formed state and the first transport route is formed when the medium receiving tray comes into the medium receiving state.