Abstract: A plate-type heat exchanger is constructed by piling a plurality of heat transfer plates (P3) having an aspect ratio (=(longitudinal length (Y))/(lateral length (X))) of 1.5. The heat transfer plate (P3) is formed of a substantially planar plate in rectangular shape and has wave-shaped heat transfer enhancement surfaces (20a), (30a) formed on its surfaces. The four corners of the heat transfer plate, i.e., the lower left corner, the upper right corner, the upper left corner and the lower right corner, are formed with a first opening (21a) as an inlet of a first flow channel, a second opening (22a) as an outlet of the first flow channel, a third opening (23a) as an inlet of a second flow channel and a fourth opening (24a) as an outlet of the second flow channel, respectively. Around the openings (21a) through (24a), respective seals (12a) through (15a) are provided to rise toward the front side or the back side of the heat transfer plate.