PURPOSE: To prevent an outside resin layer which is formed on an inside resin layer by injection molding from being warped in a cassette half in double-layer structure.

CONSTITUTION: Cassette halves 2 and 3 are constituted of an inside resin layer 4 and an outside resin layer 5 which is subjected to injection molding by superposing it on the outside surface of the inside resin layer 4. The above outside resin layer 5 is provided with a thin-wall part 6 which is overlapped on the inside resin layer 4 and a thick-wall part 7 continuing to the thin-wall part 6. A gradual thickness-increasing thick-wall part 8 which increases its thickness gradually from the thin-wall part 6 toward the thick-wall part 7 is provided in the boundary part between the thin-wall part 6 and the thick-wall part 7 of the above outside rein layer 5.