A process for welding a pair of thermoplastic workpieces by hot gas ejecting, which includes the steps of (a) positioning a pair of works to be welded in opposed relation with each other, (b) positioning heat blast nozzles between the works, (c) ejecting heat blast from the nozzles to the opposed surfaces of the works, (d) retracting the nozzles from the position between the works (e) advancing one of the works to be in pressure contact with the other, and (f) pressurizing the two works for firmly combining the two.