In the electric-resistance-welded pipe welding apparatus, after a metal strip that is traveling is bent into a cylindrical shape by rolls so that both ends in a width direction of the metal strip face each other, a power supply portion of an induction heating device or an energization heating device is provided immediately near the metal strip which is bent into the cylindrical shape, a joule heating is performed with respect to the both ends by a power supplied from the power supply portion, thereafter, and the both ends are welded while being pressed to and coming in contact with each other. The electric-resistance-welded pipe welding apparatus includes a ferromagnetic body that is movably inserted between the both ends at a position further to the upstream than the power supply portion when viewed along the traveling direction of the metal strip, the position corresponding to an opening portion between the both ends which face each other.