To provide a heat exchanger, performing enough cooling as well according to the temperature of an operating fluid in addition to warming up of the operating fluid of an engine or an automatic transmission.

In this heat exchanger, a heat exchange part 110 for performing heat exchange between cooling water of the engine 10 and the operating fluid is disposed in a circulating passage 120A where the operating fluid of the engine 10 or the automatic transmission 20 is circulated outside, and warming up of the operating fluid after the start of the engine 10 and cooling of the operating fluid after warming up are performed by cooling water. The circulating passage 120A is provided with an auxiliary heat exchange part 140 for cooling the operating fluid, a bypass passage 124 bypassing the auxiliary heat exchange part 140 and a flow control valve 150 for controlling the flow rate of the operating fluid to the bypass passage 124 side or the auxiliary heat exchange part 140 side according to the temperature of the operating fluid after warming up.