To prevent the light of fluorescent lamps from appearing in relief as stripe patterns and to reduce the thickness of an internally illuminated signboard by using a light quantity adjusting film for limiting the direct irradiation with the fluorescent lamps.

The light quantity adjusting film for the box-shaped internally illuminated signboard (1), into which the fluorescent lamps (2) are built, consists of a cylindrical transparent sheet covered with a covering material (3). The covering material (3) is provided with light diffusion preventive materials (4) in at least the positions on the front surface side of the signboard where the fluorescent lamps come into contact. The covering material (3) is preferably polyester having a heat resistance and rigidity. The members (4) are preferably a number of surfaces deposited with fine wire aluminum by evaporation or printed surfaces in the form of a fine wire, net or grid or their combinations.