PURPOSE: To prevent erroneous loading of a film by providing the lower face part provided on the lower part of a sheet film and a pair of side face parts which rise from both side edges of the lower face part, on a doubling plate and making them symmetrical substantially.

CONSTITUTION: A doubling plate 14 formed by a ball plate, etc. contains the lower face part 18, and in both side edge side parts of this lower face part 18, a pair of side face parts 20a, 20b which are directed and rise in the vertical upper direction are formed. These side face parts 20a, 20b are placed at the side of end parts 16a, 16b of a light shielding member 12, respectively. In this case, one side face part 20a and the other side face part 20b are formed so as to be symmetrical substantially as a whole by equalizing the height, length, etc. In the center of the upper end part of these side face parts 20a, 20b, tongues 22a, 22b are provided so as to be directed in the direction for approaching each other. In such a state, a film F is laminated and contained between the lower face part 18 of the doubling plate 14 and the tongues 22a, 22b. In such a way, erroneous loading of the film F can be prevented.