A sheet bonding device includes a pressurization mold 25 which is constituted of an upper mold 27 and a lower mold 28 and performs four-side sealing through heating by sandwiching a four-side sealed portion of a bag main body 11 and a four-side sealed portion of a gas barrier function sheet 12 with pressurization surfaces; a plurality of support pins 30 which are provided in the lower mold 28 so as to be retractable and position the gas barrier function sheet 12 with respect to one surface of the bag main body 11 by penetrating the four-side sealed portion of the bag main body 11 and the four-side sealed portion of the gas barrier function sheet 12; and gas spray means 29 for making the gas barrier function sheet 12 float by spraying inert gas to an area-enlarged portion 12S, which does not overlap the bag main body 11, of the gas barrier function sheet 12 supported by the support pins 30.