When resin pipes are connected through induction heating, the resin pipes of this invention do not form inside bulges due to air and resin expansion during the heating process. Cylindrical heater 2 made of stainless steel is placed around the joint between two resin pipes 1a, 1b made of polyvinylidene fluoride. Heater 2 is then heated by induction heating to weld the joint of resin pipes 1a, 1b. Heater 2 has a plurality of holes 2a on the cylindrical surface so that air and molten resin located between heater 2 and resin pipes 1a, 1b are released through holes 2a.