A plate heat exchanger realizing improved heat exchange performance by increasing the fluidity of fluids and by promoting turbulence of the fluids, including: heat exchange elements stacked by being laid one on top of another and individually formed by assembling upper and lower plates, with an internal flow channel defined in each of the heat exchange elements and an external flow channel defined between the heat exchange elements, the internal and external flow channels allowing internal and external fluids to pass therethrough, respectively, wherein the upper and lower plates are provided with respective wave patterns having ridges and valleys, each of the heat exchange elements has an inlet port and an outlet port, the upper and lower plates respectively have an upper flange and a lower flange which are assembled with each other through fitting, and first and second flat parts are formed around the upper and lower flanges.