To provide a heat exchanger having a structure ensuring heater performance, in the heat exchanger for transmission oil, connected to a heat exchanger for a heater in series in an engine cooling water passage and the like.

In this heat exchanger 1 including an oil passage 210 for circulating the oil and the cooling water passage 220 for circulating the cooling water, and exchanging heat between the oil and the cooling water, a communication passage 260 for communicating with a water inlet 3a and a water outlet 3b of the cooling water passage, is formed on a plate 26 forming part of the oil passage. Forming the communication passage in this way, the communication passage (cooling water passage) is functioned as a bypass water passage of low water passing resistance. Accordingly, as the water passing amount of the cooling water to a heater side can be increased by applying this heat exchanger for the oil in the engine cooling water passage connecting the heat exchanger for the heater and the heat exchanger for the transmission oil in series, the heater performance can be ensured.