To prevent offset sticking of a mask sheet when an instant film is produced.

A mask sheet 5 is produced by dry laminating using a PET sheet 13 and a VMPET sheet 14. The PET sheet 13 coated with an adhesive 36 and the VMPET sheet 14 are stuck together by hot pressing at a sticking section 27. A load of 5 kg is applied to the PET sheet 13, a load of 8 kg is applied to the VMPET sheet 14, and by this load difference the produced mask sheet 5 is curled in the width direction of the sheet 5 with the PET sheet 13 inward. When an instant film is produced, since the mask sheet 5 is conveyed with the PET side downward, edge parts of the mask sheet 5 are prevented from running upon guide chutes formed in a guide rail and from being crushed by the guide chutes.