PURPOSE: To enable the welding manufacture of an electric welded steel pipe of small diameter by forming plural small holes on the peripheral wall of an impeder case.

CONSTITUTION: When a high frequency current is passed to a heating coil, a reciprocating current is generated on the part 1a to be seamed in about V shape of a metal blank stock 1 and it is locally heated at high temp. by proximity effect and skin effect. The heated part 1a to be seamed is joined by a squeezing roller. Simultaneously with this a cooling liquid is fed at high speed to the inner part thereof from the opening of one part of an impeder case 11. In this case, almost of all cooling liquid becomes going out of a small hole 12 and a large amount of cooling liquids go out of the space part of the front side apart from a joining point 1b of the part 1a to be seamed of the metal blank 1. The amt. of the cooling fluid coming out of the vicinity of the joining point 1b becomes in a small amt. to improve the cooling for the impeder 5 and the welding quality can be improved.