PURPOSE: To permit to develop a predetermind capacity with small pumping power by a method wherein plates, having comparatively short heat-transfer length, are utilized and liquid is injected so as to form a thin film on the heat- transfer surface.

CONSTITUTION: First path B, closed to the supply port 4 and discharge port 5 of heated fluid, as well as the second path A, communicating the supply port 4 of the heated fluid with the discharging port 5, are provided alternately. Fluid, injected from a nozzle 16 into the first path B, flows down on the heat-transfer surface of the plate 9 in the shape of a thin film and deprives heat from the heated fluid flowing through the neighboring second path A and evaporates in this case. Vapor, produced therein, flows to the upper vapor discharging port 2 from the lower opening 15 through a vapor introducing space D. Liquid, not vaporised yet or condensed liquid, is circulated to the nozzle 16 from a condensed liquid discharging port 3 by a liquid circulating pump 19. The plate 9 is rectangular substantially and is equipped with the heat-transfer surface of short heat-transfer length.