PURPOSE: To simultaneously weld all grid points by preparing a switching device which brings electrode pairs matched to meshes of wire grids into contact from both sides and becomes floating with respect to potential.

CONSTITUTION: Contact electrode pairs 4.1.1/4.2.1, 4.1.2/4.2.2-4.1.5/4.2.5 are provided in respective cross points of plural longitudinal wires 2 crossing orthogonally to plural transverse wires 1. These electrode pairs come into contact with those grid points from both sides, and a switching device is constituted so that juxtaposed welding electrode pairs 4.1.1/4.2.1 and 4.1.2/4.2.2 become floating with respect to potential. Respective contact electrode pairs 4.1.1/4.2.1-4.1.5/4.2.5 are included in a secondary circuit which is not fixed with respect to the potential of respective welding transformers 3.1-3.5.