Fuel cell system

A fuel cell system comprising:

• a water electrolysis device configured to electrolyze water;
• a first gas-liquid separator configured to separate a fluid discharged from the water electrolysis device into a hydrogen gas and water to be resupplied to the water electrolysis device;
• an oxygen tank configured to store an oxygen gas having been produced by water electrolysis and having been pressurized;
• a fuel cell configured to generate electric power using the oxygen gas having been produced by the water electrolysis and having been pressurized;
• a second gas-liquid separator configured to store water in a mixed gas including an unreacted oxygen gas discharged from the fuel cell;
• a water level sensor configured to detect a level of the water stored in the first gas-liquid separator;
• and
• one or more processors that execute computer-executable instructions stored in a memory, wherein the one or more processors execute the computer-executable instructions to cause the fuel cell system to:
• supply to the second gas-liquid separator the oxygen gas pressurized to have a pressure higher than an internal pressure of the first gas-liquid separator when the level of water becomes lower than a predetermined threshold value, regardless of a state of the fuel cell;
• and
• open a first on-off valve disposed on the water supply path connecting the first gas-liquid separator to the second gas-liquid separator.