The exchanger comprises thin, flat liquid-flowing passages and thin, flat air-flowing passages which are alternately interposed between the adjacent liquid-flowing passages. Each of the liquid-flowing passages is separated from each of the air-flowing passages by a heat exchanging partition plate made of a metallic plate or a synthetic resin plate so as not to have air contact with liquid passed through. Each adjacent two heat exchanging partition plates constitutes a single heat exchanger unit by connecting the upper parts of the two adjacent partition plates. A number of the heat exchanger units are vertically arranged in parallel to each other in a casing to form liquid-flowing passages between the adjacent heat exchanger units.
