A cassette (12) for storing a gas detection tape (16) consists of a cassette main body (21), and a cover plate (22) covering the back of the cassette main body (21). An unused detection paper storing section (24) for storing an unused gas detection tape (16), a used detection paper storing section (25) for storing a used gas detection tape (16), a detection paper passage (26) for connecting these storing sections (24, 25), and an exposing portion (43) for exposing a part of the detection paper passage (26) to the outside of the cassette are formed in the cassette main body (21). Periphery of the unused detection paper storing section (24) is sealed by an O-ring (80), and the portion of the passage in the upstream of the exposing portion (43) is sealed by a sealing means (85), a roller (71), a sealing portion (96), and the like.