A sheet storage cassette includes a cassette body, a cassette cover, and a lock mechanism. The lock mechanism includes a lock member swingably provided on a back-surface side of the cassette cover and engageable with a locking pin provided in the cassette insertion portion, and a biasing member biasing the lock member in a direction toward engagement with the locking pin. Along with an operation of inserting the cassette body into the cassette insertion portion, the lock member engages with the locking pin. In a state where the cassette body is inserted in the cassette insertion portion, by inserting a thin-plate shaped release member into a gap between the cassette cover and an exterior member arranged under and adjacent to the cassette cover, the lock member is caused to swing to release the engagement with the locking pin.