PURPOSE: To prevent a wire material grid from loosening by preparatorily bending a frame which bends by the shrinkage force of the grid wire, and generating reaction force to the shrinkage force with the restoring force of the bent grid in attaching the grid to a frame of a color cathode tube.

CONSTITUTION: A wire material grid 4 is aligned on a top face 3 of a main beam 1 by means of an aligning plate 5 at a constant pitch. A weight 11 is suspended from the grid 4 through a pulley 10. Then, the main beam 1 is transformed by clasping a plunge bolt 18 of a support table 17. This transformation amount δ is set to be equal to a bend amount of the center part of the main beam 1 occurring in the cooling process of the wire material grid 4 after heating. Then, the grid is heated by hot air by means of a heater 23. The temperature of the hot air is arranged to get gradually higher at both ends than the center. A welding current is applied by means of an electrode rod 7 to fix the grid to a frame and cool it. The push bolt 18 is loosened. Thus, tension can be produced in the wire material grid 4 due to shrinkage force.