Instant film unit

A peel-apart type instant film unit comprising an image receiving sheet separably adhered in a heat sealing manner to a mask member which is peeled apart from the former after the development thereof. There is provided therebetween a weak sealing layer comprising an adhesive layer of a thermoplastic polymer having hydrophilic groups and a separating layer which is alkali resistant and has a good separation property relative to the adhesive layer. The weak sealing layer can provide an easy separation between the separating and adhesive layers so as perfectly to transfer the adhesive layer to the image receiving sheet to provide a fine appearance and a writeable image frame on the image receiving sheet. The mask member is made of any polyethylene compounded paper sheets and is provided with a processing liquid-resisting layer inside the weak sealing layer for the purpose of preventing the permeation of processing liquid thereto.