PURPOSE: To simplify the welding word for a billet for manufacturing a multiple pipe and to improve the yield of a product by sealing a gap between each stock of the multiple pipe by a front end part, welding a cover for closing up tightly the gap to a rear end part, and containing an annular addition piece in the cover.

CONSTITUTION: A multiple pipe is manufactured by extruding a billet. Its billet for manufacturing the multiple pipe is constituted of an outer pipe stock 1 and an inner pipe stock 2 for constituting the multiple pipe. In such a state, a front end part of a gap G of both the stocks 1, 2 is brought to seal welding 3. In the rear end part, the gap G is closed up tightly by bringing a cover 4 to seal welding 5A, 5B. In the inside of the cover 4, an annular addition piece 6 is contained. In such a way, a welding work for a billet for manufacturing the multiple pipe is simplified, and also, the yield of a product can be improved.