A film sheath, holding a sheet film to constitute a sheet film unit, is formed from a plastic resin, and is provided with at least a tapered portion in an end fringe which is directed toward a leading end of a sliding lid when the sliding lid is moved back to a closed position in a sheet film pack containing a stack of such sheet film units, so as to provide a sufficiently large gap between two of the stacked sheet film units, for smooth thrusting of the sliding lid into between the two sheet film units. The film sheath also has a protuberance in a corner of an open end, as a stop member for stopping the sheet film from slipping out of the film sheath.