A heat exchanger for a hot water heater, the heat exchanger comprising: an outer box that partitions a first passageway, through which a first fluid circulates, by using a tube shaped peripheral wall comprising a first sidewall; a heat exchanging body housed inside the first passageway of the outer box and defining a second passageway through which a second fluid circulates, the heat exchanging body performs heat exchange between the first fluid and the second fluid; a supply pipe that protrudes from the heat exchanging body through the first sidewall and supplies the second fluid to the second passageway; a discharge pipe that protrudes from the heat exchanging body through the first sidewall and discharges the second fluid from the second passageway; a plurality of insertion holes (31, 32) formed such that they pass through the first sidewall in a first direction, and through which the supply pipe and the discharge pipe protrude to the outside of the outer box; wherein, when in use, the first fluid is combustion exhaust gas and the second fluid is water; characterized by first and second elastic seal members (40; 240) provided between the first sidewall and the heat exchanging body and that block potential leakage paths (LP1; LP2) extending from the first passageway to the outside of the outer box via the insertion holes (31, 32); and at least one biasing member (50; 250) that causes the elastic seal members (40; 240) to undergo compressive deformation by exerting a biasing force (Fl; F2) on the elastic seal members (40; 240) in a direction that is parallel to the first direction, wherein the supply pipe and the discharge pipe are respectively inserted through the first and second elastic seal members (40; 240). an outer box that partitions a first passageway, through which a first fluid circulates, by using a tube shaped peripheral wall comprising a first sidewall; a heat exchanging body housed inside the first passageway of the outer box and defining a second passageway through which a second fluid circulates, the heat exchanging body performs heat exchange between the first fluid and the second fluid; a supply pipe that protrudes from the heat exchanging body through the first sidewall and supplies the second fluid to the second passageway; a discharge pipe that protrudes from the heat exchanging body through the first sidewall and discharges the second fluid from the second passageway; a plurality of insertion holes (31, 32) formed such that they pass through the first sidewall in a first direction, and through which the supply pipe and the discharge pipe protrude to the outside of the outer box; wherein, when in use, the first fluid is combustion exhaust gas and the second fluid is water; characterized by first and second elastic seal members (40; 240) provided between the first sidewall and the heat exchanging body and that block potential leakage paths (LP1; LP2) extending from the first passageway to the outside of the outer box via the insertion holes (31, 32); and at least one biasing member (50; 250) that causes the elastic seal members (40; 240) to undergo compressive deformation by exerting a biasing force (Fl; F2) on the elastic seal members (40; 240) in a direction that is parallel to the first direction, wherein the supply pipe and the discharge pipe are respectively inserted through the first and second elastic seal members (40; 240).