PURPOSE: To efficiently perform a uniform welding by welding the end part and central part of each pipe material to be welded by heating them respectively with three pieces of induced heating coil from the inner and outer peripheral surfaces of welding surface to be lap welded.

CONSTITUTION: The second pipe material 2 of smaller diameter is inserted into the first pipe material 1 of larger diameter and they are put in layers. Outer periphery heating coil 4 is installed on the outer peripheral surface of outer pipe at welding part 3 of both pipe materials so that the coil may surround the central part of welding part 3, and inner periphery heating coils 5 and 6 are installed in the inner pipe material so that they may be located at the end of each pipe material. Then a high frequency current is supplied to heating coils 4, 5 and 6 and welding part of the first pipe material 1 and the second pipe material 2 is heated electromagnetically, and then a high frequency current of large current is instantaneously impressed on each coil and welding is performed utilizing the electromagnetic welding pressure produced by the induced current.