PURPOSE: To maintain grid tension as applied by applying tension to each wire material grid, and applying higher temperature to the regions closer to both ends and lower temperature to the center region.

CONSTITUTION: A grid wire 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 each grid 4 through a pulley 10. Then the grid is heated by hot air 24 by means of a heater 23. The temperature of the hot air is arranged to get gradually higher at both end regions than the center region so that the grid wire 4 can be expanded longer at both end regions than at the center region. Then, a welding current is applied by means of an electrode rod 7 to fix the grid to a frame and cool it. Thus, larger shrinkage force of the grid wire 4 can be obtained at both end regions than at the center region.