To provide a ribbon cassette that can quickly take up a ribbon even if the ribbon loosens by accident during nonuse, unerringly prevents a reverse rotation irrespective of the amount of taking up, is downsized, and prevents leakage of secrets by making an intentional extraction of the ribbon after printing when the ribbon is used for a take-up core difficult.

The ribbon cassette is structured so as to form a single-serration portion in the entire or a part of the end face of a core, form an engagement portion engaging with the serration portion of the end face of the core in an antikickback member, and engage the serration receiving portion of the antikickback member with the serration portion of the core. In the ribbon cassette, when not mounted on a printer, the engagement of the serration portion of the core with the engagement portion of the antikickback member makes the core rotatable in one direction and when mounted on the printer, the core is made to freely rotate by separating the serration receiving portion of the antikickback member from the serration portion of the core.