PURPOSE: To prevent the generation of penetrators and to improve welding quality by sticking zinc powder on the side end faces of a metallic strip or an open pipe, then welding said faces.

CONSTITUTION: A vessel 71 which contains viscous liq. mixed with zinc powder and is provided with an opening 71a for discharging said viscous liq. in the bottom part is provided above an open pipe OP located between an induction heating coil and the final fin pass roll. The viscous liq. is dropped through the openings 73l, 73r to the end faces 8l, 8r on the opposite sides of the pipe OP, and the pipe OP stuck with the zinc powder in the viscous liq. on the side end faces is fed into the induction heating coil. The pipe OP heated at the end faces 8l, 8r by the induction current is pressurized from both sides by squeeze rolls, and is welded at weld points, whereby it is made into the pipe.