To readily and reliably weld plates together and to readily vacuum from between the plates and to inject fluid into a swelled part in production of a heat exchanger in which the plates are superposed and welded together, and thereafter the fluid is injected into a predetermined region, which is swelled and deformed to form a flow path.

In a peripheral welding step, a plurality of the plates 2, 3 are superposed and the full periphery is welded at an outer peripheral end face to form a peripheral welded part 17. In a subsequent pressure reduction step, air remaining in a gap between the plates is suctioned and discharged from openings 11, 12 previously provided to surfaces of the plates. In a subsequent interior welding step, the plates are welded together to form an interior welded part 6 at the surfaces of the plates in a state where the gap between the plates is kept under reduced pressure, and the plates are divided into a swelled part 4 and a non-swelled part 5. In a subsequent swelling step, the fluid is injected into the swelled part 4, which is then swelled and deformed.