TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: A state detector (10) detects a physical quantity with respect to the state of the fuel cell system, wherein the change device changes the operation characteristic of the variable pressure regulator based on the detected physical quantity. The change device determines whether a foreign substance is accumulated in the fuel gas supply passage based on the detected physical quantity. When the change device determines that the foreign substance is accumulated in the fuel gas supply passage, and the foreign substance is required to be discharged, the change device changes the operation characteristic of the variable pressure regulator so that a discharge pressure of the variable pressure regulator increases. The change device determines whether the fuel cell is in a state upon start-up or resumption from an intermittent operation based on the detected physical quantity. When the change device determines that the fuel cell is in the state upon start-up or resumption from the intermittent operation, the change device changes the operation characteristic of the variable pressure regulator so that a discharge pressure of the variable pressure regulator increases. When the fuel cell system is in a state where a fuel gas leakage is detected and the fuel gas leakage is to be detected, the change device changes the operation characteristic of the variable pressure regulator so that a discharge pressure of the variable pressure regulator increases. The variable pressure regulator is structured to be able to adjust the discharge pressure by introducing a pressurized fluid from outside of the variable pressure regulator. The change device includes a pressurized fluid supply passage (31). An exhaust valve is capable of discharging the pressurized fluid from an intermediate portion of the pressurized fluid supply passage to outside of the pressurized fluid supply passage. A controller controls an opening degree of the exhaust valve in accordance with the state of the fuel cell system.