To maintain the flatness of sheet film by suppressing the projecting deformation of the sheet film in spite of exposure for a long time under the environment of high humidity.

One sheet film 3 is housed in a sheath made of resin 2 at a film unit 1. The sheath 2 is constituted of a rectangular plate part 2a supporting the base surface 3g of the sheet film 3 and edge parts 2b-2d which are formed at three sides of the plate part 2a and possess grooves where the edges of three sides of the sheet film are inserted. The end parts 6a of hood-like parts 6 constituting the grooves of the edge parts 2b-2d are projected on the side of the plate part 2a, and an interval between the end part 6a and the plate part 2a is set to 0.3mm, for instance, for the sheet film 3 having 0.24mm thickness. Thus, the sheet film 3 is pressed by the end parts 6a of the hood-like parts 6, and the edges of the sheet film 3 are inserted into the grooves of the edge part 2b-2d without troubles.