Abstract: A method of joining coated large-diameter steel pipes laid underground which comprises placing the steel pipes end to end, applying a layer of heat insulating material to the outer surface of the welding part, covering the ends of the pipe coating the non-coated part of the pipe ends and the layer of the heat insulating material with a layer of a heat fusible, heat adhesive, corrosion resisting material of a shape corresponding to the external shape of the pipes, placing a thin clamp plate around the outer surface of this layer to make it fast, and then internally welding the groove to join the pipes together, whereby the welding heat is utilized effectively and both the corrosion resisting material and the pipe coating are fused by the heat, thus causing the thermally fused corrosion resisting material to adher and solidify on the uncoated portions of the pipes and unite with the ends of the pipe coating and thereby forming a corrosion resisting covering layer.