To provide a method for welding a plate-like strip material, capable of preventing fracture when deformed-cross-section-working is applied to a coil welding part, capable of improving productivity of a deformed cross section strip material, and capable of reducing the manufacturing cost.

A deformed cross-section strip material manufacturing device 50 is used which includes: a flat V-shaped die 40 having protrusions 41a and 41b whose rolling work surface is widened toward the end from a tip side in V-shape; and a planetary rolling roll 51 that is provided to face the rolling work surface of the flat V-shaped die 40. When the plate-like strip material 53 is introduced between the flat V-shaped die 40 and the planetary rolling roll 51 from a coil 54 to which the plate-like strip material 53 in the prescribed length is wound, and the plate-like strip material 53 is continuously rolled to the deformed cross section strip material 62, strip material end surfaces of the plate-like strip material 53 of a preceding coil 54 before replacement and a succeeding coil 11 after replacement are cut in a shape to be fitted with each other during replacement of the coils, and the strip material end surfaces cut in the fitted shape are fitted with each other, and then, welding is performed.