A medium storage device that is covered by a dust cover and that stores a recording medium, includes: a guide member that is extendably and retractably provided in the medium storage device and that restricts a position of an edge part of the recording medium; and a lock member that locks the guide member and that releases the lock status of the guide member when the lock member moves in a first direction so as to allow the guide member to extend and retract, wherein the movement of the lock member in the first direction is restricted by the dust cover so that the guide member is stably locked.