Welding joining method and welding joined body

A welding joining method for joining end portions of tubular members made of crystalline resin by bonding the end portions to each other by pressure in a molten state, the welding joining method comprising:

• preparing an infrared emission unit configured to change characteristics of infrared to be emitted;
• a placing step of placing the infrared emission unit between the end portions of the tubular members, the end portions being placed to face each other at an interval in a tubular axial direction;
• a heating and melting step of heating and melting the end portions of the tubular members by emitting the infrared from the infrared emission unit;
• and
• a pressure bonding step of cooling down the molten end portions of the tubular members in a state where the end portions are bonded to each other by pressure, wherein:
• in the heating and melting step, the characteristics of the infrared emitted from the infrared emission unit are controlled so that a crystallinity along a tubular radial direction in a region starting from a boundary portion in a welding joined body after cooling does not suddenly change, the boundary portion being a part between a peripheral surface of a corresponding one of the tubular members and a weld bead discharged from a joined portion in pressure bonding such that the weld bead projects in the tubular radial direction.