An ion exchanger for a cooling system of a fuel cell system includes: a communicating pipe part with the two ends thereof configured respectively to be connectable to predetermined piping arranged in the cooling system, and including a substantially linear shaped first flow path; and a storage case including a second flow path configured such that, when a part of the coolant introduced into the communicating pipe part branches from the coolant of the communicating pipe part and flows toward the second flow path, such partial coolant, after introduced into the second flow path, is allowed to flow therethrough and join again the remaining coolant of the communicating pipe part, while ion exchange resin is stored in the second flow path, wherein an introduction device for introducing the coolant into the second flow path is arranged in the inside of the communicating pipe part.