A rack includes: a casing; M (an integer not less than three) pairs of supports that are provided inside the casing so as to extend in a depth direction and arrayed at regular intervals in a predetermined direction (a height direction or a left-right direction); and a lock mechanism. The rack can house all returnable containers of one or more prespecified sizes along the respective pairs of supports. The lock mechanism includes N (an integer not less than two and meeting N