PURPOSE: To enhance a welding strength of an end plug to a tube.

CONSTITUTION: An end plug 12 is formed in a virtually columnar shape having a slightly larger outside diameter than the one of a cladding tube 1 and a tapered contact face 14 is provided on the outer peripheral side of a fore end face 12a thereof on the cladding tube 1 side. When the fore end face 12a of the end plug 12 is brought into contact with the cladding tube 1 concentrically substantially, the contact face 14 is brought into ring-shaped linear contact with an outer peripheral edge part 1c of the cladding tube 1. When a pair of electrodes 4 and 5 are brought into pressure contact with the outer peripheral surface of the cladding tube 1 oppositely and electrified, a current (i) at the time of start of welding flows at the shortest distance along the outer peripheral surface of the cladding tube 1 from the electrode and runs to the end plug 12 through a welding part wherein the outer peripheral edge part 1c and the contact face 14 are in pressure contact with each other, so that the welding be performed.