A heat exchanger comprising: 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, wherein the heat exchanger body comprises: 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, the wave fins having a wave fin of a U - shaped cross-section and a wave fin of a ∩-shaped cross-section that are arranged in series along a width direction of the wave fin plate, wherein each of the wave fins includes a fixed pitch section adjacent to a position of the gas inlet, and a variable pitch section adjacent to a position of the gas outlet along a longitudinal direction of the wave fin, each of pitches (b, c) within the variable pitch section of the wave fin being always greater than each of pitches (a) within the fixed pitch section of the wave fin, characterised by the variable pitch section occupying 10 to 60% of a total length of the wave fin plate, wherein in each of the wave fins, a first pitch (b) of the variable pitch section is 1.1 to 2.5 times greater than a fixed pitch (a) of the fixed pitch section, wherein the variable pitch section includes a plurality of sections in which a pitch gradually increases by 1.2 to 1.8 times greater than a pitch of a preceding section. 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, wherein the heat exchanger body comprises: 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, the wave fins having a wave fin of a U - shaped cross-section and a wave fin of a ∩-shaped cross-section that are arranged in series along a width direction of the wave fin plate, wherein each of the wave fins includes a fixed pitch section adjacent to a position of the gas inlet, and a variable pitch section adjacent to a position of the gas outlet along a longitudinal direction of the wave fin, each of pitches (b, c) within the variable pitch section of the wave fin being always greater than each of pitches (a) within the fixed pitch section of the wave fin, characterised by the variable pitch section occupying 10 to 60% of a total length of the wave fin plate, wherein in each of the wave fins, a first pitch (b) of the variable pitch section is 1.1 to 2.5 times greater than a fixed pitch (a) of the fixed pitch section, wherein the variable pitch section includes a plurality of sections in which a pitch gradually increases by 1.2 to 1.8 times greater than a pitch of a preceding section. 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, the wave fins having a wave fin of a U - shaped cross-section and a wave fin of a ∩-shaped cross-section that are arranged in series along a width direction of the wave fin plate, wherein each of the wave fins includes a fixed pitch section adjacent to a position of the gas inlet, and a variable pitch section adjacent to a position of the gas outlet along a longitudinal direction of the wave fin, each of pitches (b, c) within the variable pitch section of the wave fin being always greater than each of pitches (a) within the fixed pitch section of the wave fin, characterised by the variable pitch section occupying 10 to 60% of a total length of the wave fin plate, wherein in each of the wave fins, a first pitch (b) of the variable pitch section is 1.1 to 2.5 times greater than a fixed pitch (a) of the fixed pitch section, wherein the variable pitch section includes a plurality of sections in which a pitch gradually increases by 1.2 to 1.8 times greater than a pitch of a preceding section.