Disclosed is a heat exchanger including: a heat exchanger body; a gas inlet for introducing exhaust gas into the heat exchanger body; a coolant inlet for introducing a coolant into the heat exchanger body; a gas outlet for discharging the exhaust gas that is cooled by heat exchange with the coolant; and a coolant outlet for discharging the coolant that completes heat exchange with the exhaust gas. In this case, the heat exchanger body includes: a laminated tube core formed by laminating a plurality of gas channels side by side; a housing formed so as to enclose the laminated tube core except for opposite ends thereof; and a wave fin plate integrally provided with a plurality of wave fins and arranged within each of the gas channels, wherein each of the wave fins includes a fixed pitch section, and a variable pitch section.