Provided is a cover piece to be removably attached to a medium storage cassette including a medium storage section for storing a medium, and an operation unit provided in a recess and configured to lock and unlock the medium storage section to and from an apparatus main body. The cover piece includes a main portion configured to enter the recess in the medium storage cassette so as to cover the recess, and a sliding portion configured to slide so as to switch between a first state in which the sliding portion sticks out from the main portion and the main portion is locked to the recess, and a second state in which the main portion is unlocked and allowed to enter the recess and be removed from the recess.