FUEL CELL SYSTEM

A fuel cell system, wherein the fuel cell system comprises a fuel cell, an oxidant gas system and a controller; wherein the oxidant gas system comprises an oxidant gas supply flow path, an oxidant off-gas discharge flow path, a first bypass flow path, an oxidant gas supplier, a pressure control valve, a gas-liquid separator and a humidifier; wherein the oxidant gas supplier comprises, on the oxidant off-gas discharge flow path side, an expander connected to the oxidant gas supplier by an axis.