An ion exchanger includes a lower casing, an upper casing, and a cartridge. The lower casing includes an upper opening and a circumferential wall, which includes an intake port and a discharge port. The upper casing includes a lid, which is arranged on the opening of the lower casing, and a cylinder, which extends downward from the lid and is accommodated in the circumferential wall. The cartridge, which is provided integrally with the inner side of the cylinder, accommodates an ion exchange resin. The cylinder includes a communication hole, through which the inner side of the cylinder is in communication with the intake port. The upper casing includes an accumulation limiting structure that limits the air remaining immediately below the lower surface of the lid in the upper casing after flowing into the cylinder together with coolant.