A medium feeding cassette includes a cassette main body, a cassette extension member mounted to the cassette main body so as to be movable with respect to the cassette main body, and a locking mechanism configured to lock the cassette extension member at a predetermined position with respect to the cassette main body according to a size of the medium. The locking mechanism includes a locking member and a first locking groove that engage each other so as to lock the cassette extension member in the predetermined position. The locking member moves in a direction to release locking of the cassette extension member when a force is applied to the cassette extension member in a direction to retract the cassette extension member into the cassette main body.