PURPOSE: To simplify the production process of fuel pellets and compiling work of pellet stack keeping the pushing pressure of plenum spring and provide a fuel rod speedily and inexpensively by reducing the variation of the pellet stack length of fuel rods and insertion depth of fuel clad end plug.

CONSTITUTION: The insertion part 7 of an end plug 5 in a clad 1 is formed longer than the length of a fuel pellet 2 and an end plug 5 is pushed in the insertion direction with the same pressure determined by the plenum spring 3 and the insertion depth of the end plug insertion part 7 is automatically controlled. Also, by the insertion depth of the insertion part 7, the randomness of the pellet stack length S due to a maltitude of fuel pellets 2 is adjusted.