PURPOSE: To prevent a thermal feedback by measuring the stress of a cladding pipe in the stage of introducing a high-pressure inert gas into the fuel rod formed by inserting a fuel pellet into a cladding pipe and welding the vent hole of the cladding pipe when the measured value is a specified value or above, thereby preventing the vent hole from being sealed hermetically in an unsatisfactorily pressurized state.

CONSTITUTION: The stress condition of a cladding pipe for a fuel rod (n) which is held unpressurized is measured by an X-ray device 17 for measuring stress and is recorded in a deviation measuring device 18. The air in a pressurized vessel 11 into which the part of an end plug 5a with a vent hole of the rod (n) is inserted is expelled from the vessel and an inert gas kept under high pressure is introduced into the vessel. The pressure in the vessel 11 is measured in this state and the stress of the cladding pipe is measured by the device 17. The measured values are compared with an initial stress value by the device 18 and the signal attaining the prescribed deviation value and the signal attaining the prescribed pressure of the vessel 11 are inputted to a control device 13, then the vent hole of the plug 5a is welded.