To prevent a plurality of lids from being mounted on one envelope one over another in a seam bonding device.

The seam bonding device which mounts a lid on an opening portion of the envelope containing an electronic component and seam-bonding the envelope and lid together to seal the opening portion includes a suction nozzle which suctionally holds and conveys the lid, a stage on which the lid sucked by the suction nozzle is temporarily mounted before being mounted on the envelope, a suction nozzle holding mechanism which lowers the suction nozzle when mounting the lid on the stage and carries on the lowering operation further by a predetermined distance even after a tip of the suction nozzle sucking the lid abuts against a top surface of the stage with the lid interposed, and a measuring instrument which measures the quantity of a relative position shift between the suction nozzle holding mechanism and suction nozzle and judging one or plural of lids based on the measuring result.