In a film cassette, a film spool comprises a spool core supported for rotation in an unwinding direction to thrust a leader section of a film roll coiled about the spool core to the exterior of the cassette, a pair of flanges radially confining the film roll and loosely coupled coaxially with the spool core normally to permit the spool core to be rotated relative to the flanges in the unwinding direction, and engagement means rotatably fixed to the spool core for rotation with the spool core relative to the flanges in the unwinding direction to move into engagement with one of the flanges to make the one flange rotate in unity with the spool core in the unwinding direction. According to the invention, the one flange is resiliently flexible, and biasing means distorts the one flange for urging the one flange firmly against the engagement means to ensure the engagement means will engage the one flange when the spool core is rotated relative to the flanges in the unwinding direction.