Provided is a welding joining method for joining end portions of first and second pipes made of polyamide resin by bonding the end portions to each other by pressure in a molten state. The welding joining method includes: a placing step of placing an infrared radiation lamp between the first and second pipes placed to face each other at an interval; a heating and melting step of heating and melting the end portions of the first and second pipes by emitting infrared; and a pressure bonding step of cooling down the molten end portions in a state where the molten end portions are bonded to each other by pressure.