A liquid film evaporation type heat exchanger including a plurality of heat transfer units, with each unit including a plurality of flat heat transfer ducts of square cross section formed with a plurality of warm water passageways extending perpendicular to the direction of flow of a liquid medium, and a plurality of liquid distributing beams formed with a pluraity of cutouts. The heat transfer ducts and the liquid distributing beams being alternately arranged in the respective units, with the heat transfer units being arranged at locations spaced apart from each other in the direction of flow of the liquid medium. Vapor releasing ports are formed in positions between the adjacent heat transfer units. Each heat transfer duct has on either side surface thereof a porous material layer providing a heat exchange surface on which a film of the liquid medium is formed for evaporation.