A recording tape cassette which accommodates a reel on which a recording tape is wound. A lock member for locking the reel is rotatably attached at an inner face of a case of the recording tape cassette. The lock member is urged to a reel side thereof, and engages with a gear which is formed at a peripheral edge of a flange of the reel. A guide surface is formed at the lock member such that, at a time when the reel is inserted into the case, the lock member is pushed by the reel and consequently is withdrawn in a direction of releasing locking of the gear. With this structure, an operation of insertion of the reel can be carried out easily, and work efficiency can be improved.