A plate heat exchanger, comprising: a plurality of heat exchange elements stacked in such a way that they are laid one on top of another, each of the heat exchange elements being formed by assembling an upper plate and a lower plate, with an internal flow channel defined in each of the heat exchange elements and allowing an internal fluid to pass therethrough, and an external flow channel defined between the heat exchange elements and allowing an external fluid to pass therethrough, wherein the upper plate is provided on an upper surface thereof with a wave pattern, the wave pattern of the upper plate comprising a plurality of ridges and a plurality of valleys, and the lower plate is provided on a lower surface thereof with a wave pattern, the wave pattern of the lower plate comprising a plurality of ridges and a plurality of valleys; each of the heat exchange elements is provided at opposite ends thereof with an inlet port and an outlet port; the upper plate has an upper flange which is raised upwards from each of the inlet and outlet ports (43, 44), and the lower plate has a lower flange which protrudes downwards from each of the inlet and outlet ports (43, 44), the upper flange and the lower flange being assembled with each other through fitting; and a first flat part is formed around each of the upper flanges of the upper plate and a second flat part is formed around each of the lower flanges of the lower plate; characterized in that the first flat part is formed on a part of an area around each of the upper flanges, and the wave pattern of the upper plate extends to another part of the area around each of the upper flanges so that the first flat part partially surrounds the associated upper flange; and the second flat part is formed on a part of an area around each of the lower flanges, and the wave pattern of the lower plate extends to another part of the area around each of the lower flanges so that the second flat part partially surrounds the associated lower flange; wherein the first flat part and the second flat part are offset from each other on each of the inlet and outlet ports (43, 44). a plurality of heat exchange elements stacked in such a way that they are laid one on top of another, each of the heat exchange elements being formed by assembling an upper plate and a lower plate, with an internal flow channel defined in each of the heat exchange elements and allowing an internal fluid to pass therethrough, and an external flow channel defined between the heat exchange elements and allowing an external fluid to pass therethrough, wherein the upper plate is provided on an upper surface thereof with a wave pattern, the wave pattern of the upper plate comprising a plurality of ridges and a plurality of valleys, and the lower plate is provided on a lower surface thereof with a wave pattern, the wave pattern of the lower plate comprising a plurality of ridges and a plurality of valleys; each of the heat exchange elements is provided at opposite ends thereof with an inlet port and an outlet port; the upper plate has an upper flange which is raised upwards from each of the inlet and outlet ports (43, 44), and the lower plate has a lower flange which protrudes downwards from each of the inlet and outlet ports (43, 44), the upper flange and the lower flange being assembled with each other through fitting; and a first flat part is formed around each of the upper flanges of the upper plate and a second flat part is formed around each of the lower flanges of the lower plate; characterized in that the first flat part is formed on a part of an area around each of the upper flanges, and the wave pattern of the upper plate extends to another part of the area around each of the upper flanges so that the first flat part partially surrounds the associated upper flange; and the second flat part is formed on a part of an area around each of the lower flanges, and the wave pattern of the lower plate extends to another part of the area around each of the lower flanges so that the second flat part partially surrounds the associated lower flange; wherein the first flat part and the second flat part are offset from each other on each of the inlet and outlet ports (43, 44).