The objective is to provide a seat storage device that can suppress adjustments to the storage position of the cassette. [Solution] The sheet storage device of the embodiment comprises a case, a cassette, a guidance mechanism, a cassette body, a cover, and a biasing member. The cassette moves out of the case by moving to the pull-out side in the
insertion / removal direction. On the other hand, the cassette moves to the
insertion side, which is opposite to the pull-out side in the
insertion / removal direction, and fits into the case. The guidance mechanism guides the cassette to the position where it fits into the case. The cassette body is included in the cassette and is capable of storing a sheet. The cover is included in the cassette and is positioned on the pull-out side of the cassette body. The cover is movable relative to the cassette body in the insertion / removal direction. The cover can contact the case when the cassette fits into the case. The biasing member is included in the cassette and biases the cover toward the insertion side toward the cassette body.