A medium storage box includes an internal storage space configured to store a paper sheet-shaped medium. The medium storage box includes: a first opening into which an insertion member is inserted from outside; a second opening through which the medium passes; a first shutter member that closes off the first opening; a second shutter member that closes off the second opening; and a biasing mechanism that biases either the first shutter member, or the second shutter member, or both, in a closing direction. The first shutter member is configured to be pressed by the insertion member and moves in an opening direction during insertion of the insertion member, such that a portion of the first shutter member abuts the second shutter member and moves the second shutter member in an opening direction.