A medium transport device includes a base frame which includes an outer edge forming portion which configures an outer edge of a device top surface, a medium support tray which is capable of switching between a horizontal posture, and a sub-tray which is capable of being displaced between a storage position and a pulled-out position, and supports the medium at the pulled-out position, in which the sub-tray includes a convex portion which configures a portion of the flat device top surface together at the storage position, and in which the sub-tray is capable of being displaced along a direction that is parallel to the device top surface in a state in which the medium support tray takes the horizontal posture.