Gas canister retaining structure includes: a gas canister cover pivotable between an opened position for attaching/detaching a cassette gas canister to/from the retaining structure and a closed position for accommodating and holding the gas canister in place within the canister cover; a mouthpiece support section provided inside the gas canister cover for supporting a mouthpiece portion of the cassette gas canister with a jet nozzle of the cassette gas canister received in a nozzle receiving port provided in the mouthpiece support section; and a shutter member pivotable between an opened position for opening the nozzle receiving port and a closed position for closing the nozzle receiving port.