A fuel cell system of the present invention includes a fixing unit that fixes or releases hydrazine, a fuel cell to which hydrazine released in the fixing unit is supplied as fuel, a supply line for supplying an aqueous hydrazine solution from a hydrazine supply source to the fixing unit, a drain line for draining drained water from the fixing unit, a first sensing unit for detecting a hydrazine concentration in an aqueous hydrazine solution flowing in the supply line, a second sensing unit for detecting a hydrazine concentration in drained water flowing in the drain line, and a detection unit that detects the amount of hydrazine fixed in the fixing unit based on the concentration values detected by the first sensing unit and the second sensing unit.