PURPOSE: To provide a tank for a heat exchanger constituted so that a partition part does not fall down at the time of the mounting of the tank and it can be judged whether the partition part is mounted at an accurate position from the outside after the tank part is mounted and the liquid in the tank part does not leak to the outside.

CONSTITUTION: A leg part 42 is integrally provided to a partition part 4 and a flat part 41 is formed to the leg part 2. Since the partition part 4 is placed on a seat plate part 2 in a self-supported state, the partition part 4 is prevented from falling after a tank part 1 is mounted. Further, a hole part 6 is provided to a header part 3 and it can be checked whether the partition part 4 is mounted at an accurate position through the hole part 6. Since the flat part 41 is provided to the leg part 42 so as to be closely bonded to the hole part 6 by soldering, the engine cooling water in the tank part 1 is prevented from leaking to the outside through the hole part 6.