A gas liquid separator of a fuel cell system includes a first gas liquid separation chamber capable of storing water content separated from an oxygen-containing exhaust gas, a second gas liquid separation chamber positioned above the first gas liquid separation chamber, and connected to the first gas liquid separation chamber, an inlet, an outlet, and a gas inlet channel for guiding a dry gas into the first gas liquid separation chamber. A gas inlet section forming the gas inlet channel includes an inner protrusion protruding into an upper space of the first gas liquid separation chamber, and a bottom surface of the first gas liquid separation chamber is inclined downward in a direction in which the inner protrusion extends.