PURPOSE: To make the manufacture of a long-sized pipe having stable quality possible by evacuating air from one end of two hollow round bars to be butt- welded with a vacuum pump, blinding the other end, containing the narrow range including the butt parts in a vacuum chamber and welding said parts while rotating the round bars synchronously.

CONSTITUTION: One end of two hollow pipes 1, 2 to be butt-welded is connected through an adapter 6 having a rotating seal to a vacuum pump, by which air is evacuated therethrough. The other end 5 is held blinded. The extremely limited range including the butt parts is contained in a welding chamber 3 via packings 4, 4' for vacuum sealing. The pipes 1, 2 are rotated synchronously with rotating mechanisms 7, 7' and while the inside of the chamber 3 is evacuated to a vacuum with an another evacuating device, the joint parts are welded by an electron beam.