PURPOSE: To perform effective welding by joining and clamping resin joined members and blowing hot air to the joint.

CONSTITUTION: The joining surfaces 1a, 2a of resin air-tightly joined members 1, 2 joined and compressed by a suitable clamping device to each other and united into an air-tight body. Hot air is blown partially to the joint J. The relation between the temperature and the pressure of the hot air and the blowing time thereof differ, originally, with resin material, and in a joined member of polybutylene terephthalate (PBT) hot air is blown at about 390°C, at 1kg/cm2, for 5min. After blowing the hot air, the member is removed out of the clamping device 9 and cooled naturally to complete welding. In this method, quick and effective welding is carried out.