The gas cell sealing method improves the sealing precision of gas cells without having a thermal load applied to the gas cell main body or to the gas inside gas cells. A gas cell sealing method includes bonding step in which a bonding portion of a gas cell main body having an opening and a lid for closing the opening where a sealant for bonding the gas cell main body and the lid is provided is heated by converging a laser beam from a laser light source through an optical element, and the lid is bonded to the gas cell main body by applying pressure in a direction that presses the lid against the gas cell main body, the pressure being applied while the laser beam is being converged to the bonding portion.