To provide a heat exchanger for a vehicle, reducing significant reduction of circulation resistance when heat exchange of a circulating medium is unnecessary.

An inlet 19 and an outlet 20 for the circulating medium are formed on positions through a partitioning section of a tank 2, the partitioning section is provided with a valve 10 changing adjacent rooms in a communicated state, and the almost total amount of the circulating medium is allowed to circulate toward the outlet 20 while bypassing all of pathways P1-P3 from the inlet 19, when the valve 10 is opened.