PURPOSE: To prevent the danger of mixing of fluids due to corrosion perforation without damaging heat transfer property, by a method wherein a supporting plate comprising the third passage separate from and independent of the first and second passages is provided between adjacent layers of a heat exchanger to constitute a partition wall for thermally connecting heat transfer walls.

CONSTITUTION: A plate 7 forming the first passage is provided with through- holes 8W13, a centeral part thereof consists of grooves 14 and ridges 15, and a fluid introduced through the hole 8 is passed through the grooves 14, and is discharged through the hole 9. A plate constituting the second fluid passage is the same as the above plate in respect of shape and the holes 8W13, except that the hole 10 is communicated with the hole 11, and the holes 8, 9 are isolated. A separating plate 16 is so constructed that passages formed by groove parts 17 and ridge parts 18 are communicated with holes 12, 13. By intermediately providing the plate 16 between the first and second plates and integrating them in a laminate form, heat transfer characteristics can be maintained, and danger of mixing of two fluids due to corrosion perforation can be prevented from occurring.