A heat exchanger is provided with a first tank and a second tank. Heat transfer tubes are disposed between the tanks and are connected to the tanks to place the tanks in fluid communication. At least one of the tanks is divided into chambers by a partition. A reinforcement plate member is disposed in at least one of the first and the second tanks for reinforcing the tank to prevent deformation of the tank due to an increase of the pressure within the tanks. The reinforcement plate member is not connected to the partition.