A medium housing device includes: a frame including an internal space; a partitioning plate that partitions the internal space so as to intersect with a specific collection direction, and that forms a collection space in which the medium is collected along the collection direction; a partitioning plate moving section that moves the partitioning plate along the collection direction; a side guide that is attached inside the frame and that determines the size of the collection space in an intersecting direction intersecting with the collection direction; a positioning portion that positions an end portion of the side guide at one attachment position selected from a plurality of attachment positions; and a retention switching portion that switches between a retained state in which the end portion of the side guide is positioned, and a retention-released state in which the end portion of the side guide is not positioned.