A heat exchanger may include a heat radiating part, in which a plurality of plates is stacked and a plurality of first flow paths and a plurality of second flow paths disposed while crossing each other are formed inside the plurality of plates, the heat exchanging part being configured to mutually heat-exchange working fluids passing through the first and second flow paths, respectively, and a sub-expansion part integrally formed with an inner side of the heat radiating part, connected with the second flow path, and configured to bypass a part of one working fluid among the working fluids flowing into the heat radiating part and heat-exchange the bypassed working fluid with the working fluid passing through the first flow path.