A self-advancing film cassette is provided with an auxiliary rotation transmittance structure for reliably transmitting rotation of a spool to the inner turns of a roll of film in order to prevent reverse rotation of the film caused by loosening of the inner turns of the roll of film or inferior bending rigidity. According to one preferred embodiment, a step portion for pushing the bending line of a film trailer is formed on a slit, permitting the end of the film trailer to be inserted therein. In another preferred embodiment, a spool is provided with a resilient member or flap member for urging the inner turns of the roll of film toward its outer turns. The film trailer has increased bending rigidity by having an embossing treatment, applied thereto, or by attaching a skirt thereto.