PURPOSE: To make sealing work of upper end plug efficient by melting with laser irradiation around a vent hole at the upper surface of an upper end plug, sucking in the vent hole and forming a projection for welding for sealing the end part.

CONSTITUTION: A vent hole 7 is formed in the axis direction of an upper end plug 5 and a projection for welding 8 is formed in ring shape in one with the same material as the end plug 5 around the hole 7 of the end plug upper surface 5a. After inserting the upper part of the fuel rod in a pressure room in a chamber and introducing helium gas of ca. 25 to 35kg/cm2, laser light is casted by way of a laser light transmission part consisting of pressure resistant glass. The projection for welding 8 is melted by the radiation of laser light and sucked in a hole 1 owing to capillary effect to block the hole 7. By this, the labor and time required for gap adjustment between electrodes and exchange of electrodes are avoided and sealing work of the upper end plug is improved in efficiency.