A heat exchanger, comprising: a heat exchanger body that includes a high-temperature channel through which a high-temperature fluid flows and a low-temperature channel through which a low-temperature fluid flows, and that exchanges heat between the high-temperature fluid flowing through the high-temperature channel and the low-temperature fluid flowing through the low-temperature channel; a high-temperature inlet pipe that is connected to an inlet of the high-temperature channel of the heat exchanger body and allows the high-temperature fluid to flow into the high-temperature channel from an outside; a low-temperature inlet pipe that is connected to an inlet of the low-temperature channel of the heat exchanger body and allows the low-temperature fluid to flow into the low-temperature channel from the outside; a flow-straightening ring that is disposed within at least either one of the high-temperature inlet pipe and the low-temperature inlet pipe and allows the high-temperature fluid or the low-temperature fluid to be mixed; and a temperature sensor provided in such a manner that a sensing point is arranged in a core region formed in a downstream region of the flow-straightening ring, characterized in that the flow-straightening ring includes an aperture that allows the high-temperature fluid or the low-temperature fluid to pass therethrough, and an opening diameter of the aperture is, on an upstream side, equal to an inside diameter of the high-temperature inlet pipe or the low-temperature inlet pipe, and is, on a downstream side, smaller than the opening diameter on the upstream side. a heat exchanger body that includes a high-temperature channel through which a high-temperature fluid flows and a low-temperature channel through which a low-temperature fluid flows, and that exchanges heat between the high-temperature fluid flowing through the high-temperature channel and the low-temperature fluid flowing through the low-temperature channel; a high-temperature inlet pipe that is connected to an inlet of the high-temperature channel of the heat exchanger body and allows the high-temperature fluid to flow into the high-temperature channel from an outside; a low-temperature inlet pipe that is connected to an inlet of the low-temperature channel of the heat exchanger body and allows the low-temperature fluid to flow into the low-temperature channel from the outside; a flow-straightening ring that is disposed within at least either one of the high-temperature inlet pipe and the low-temperature inlet pipe and allows the high-temperature fluid or the low-temperature fluid to be mixed; and a temperature sensor provided in such a manner that a sensing point is arranged in a core region formed in a downstream region of the flow-straightening ring, characterized in that the flow-straightening ring includes an aperture that allows the high-temperature fluid or the low-temperature fluid to pass therethrough, and an opening diameter of the aperture is, on an upstream side, equal to an inside diameter of the high-temperature inlet pipe or the low-temperature inlet pipe, and is, on a downstream side, smaller than the opening diameter on the upstream side. <EOS>