To provide a heat exchanger having superior heat exchanging efficiency and capable of being miniaturized.

In this heat exchanger wherein a high-temperature fluid 8 flows to the low-temperature fluid to allow the heat to be transferred therebetween, a low-temperature fluid channel accommodating the low-temperature fluid is provided at least with two temperature zones, that is, a low temperature zone and a high temperature zone higher than the low-temperature zone, a plurality of units 1, 2, 3 provided with the high-temperature fluid channels are horizontally arranged to generate the heat exchange on both of at least two temperature zones, and the high-temperature fluid channels 4 of the units 1, 2, 3 are connected in series.