A medium storage device equipped with: a discharge unit that discharges a paper-sheet-shaped medium into a storage space that stores the medium; a stacking stand, which is provided within the storage space, and which stacks the medium discharged from the discharge unit on a stacking surface inclined with respect to the vertical direction; a stacking stand moving unit that moves the stacking stand in the direction in which the medium is stacked; and stacking guides, which are attached to the stacking stand, and which support the medium by means of a stack-facing surface that faces the lower side of the medium stacked on the stacking surface, or the vicinity thereof. Downslide prevention parts, which prevent the lower side of the medium from sliding downward, are formed on stack-facing surface of the stacking guides. Rollers are provided on the stacking guides, on the opposing surface opposing the circumferential-side surface enclosing the storage space.