FUEL CELL SYSTEM

A fuel cell system (15) includes: a fuel cell stack (600); a first cooling medium circuit (505) through which a cooling medium for cooling the fuel cell stack (600) flows; an ion exchanger (540) that removes ions in the cooling medium; a second cooling medium circuit (705) in which the average ion concentration of the cooling medium is lower than that of the cooling medium in the first cooling medium circuit (505); a switching valve (740) that switches between a flow state and a low flow state; a pump (725) configured to cause the cooling medium in the second cooling medium circuit (705) to flow into the first cooling medium circuit (505); and a control unit (900) that, when a stop period of the fuel cell system (15) is longer than a reference period, drives the pump (725) with the switching valve (740) switched to the flow state after the instruction to start the fuel cell system (15) is input.