PURPOSE: To obtain a film unit capable of maintaining beautiful whiteness in the white frame area of a print even with lapse of time by masking its end with a mask containing an acid.

CONSTITUTION: A heat sensitive adhesive with an acid or a substance releasing an acid, such benzoic acid or its anhydride, contained in paper, a polymer film, or the like is formed by coating to prepare a mask, and this is laminated on the end of a film unit. Mask 4 with given perforations made and with spacer rail 6 applied is stuck to long photosensitive sheet 1, trap 5 and processing vessel 3 are successively stuck to mask 4. Cover sheet 2 is overlaid on photosensitive sheet 1, the end of mask 4 with trap 5 stuck and the end of mask 4 with vessel 3 stuck are folded back, respectively, and they are stuck to the backside of cover sheet 2. This is cut at given intervals, and thus, the film unit is obtained.