A sheet bonding device includes a pressurization mold which is constituted of an upper mold and a lower mold and performs sealing through heating by sandwiching a sealing target portion of a bag main body and a sealing target portion of a gas barrier function sheet with pressurization surfaces; a plurality of support pins which are provided in the lower mold so as to be retractable and position the gas barrier function sheet with respect to one surface of the bag main body by penetrating the sealing target portions of the bag main body and the gas barrier function sheet; and gas spray means for making the gas barrier function sheet float by spraying inert gas to an area-enlarged portion, which does not overlap the bag main body, of the gas barrier function sheet supported by the support pins.