A slide glass storage device including: a supply tray which has a supply holding space determined by a longer edge upper limit allowed for a longer edge of a slide glass and a shorter edge upper limit allowed for a shorter edge of the slide glass and on which a slide glass to be supplied to a treatment unit operable to perform a predetermined treatment is mounted; a discharge tray which has a discharge holding space determined by a longer edge longer than the longer edge upper limit and a shorter edge longer than the shorter edge upper limit and on which a slide glass having been subjected to the treatment in the treatment unit and being to be discharged is mounted; and a support unit supporting the supply tray and the discharge tray.