A gas liquid separator of a fuel cell system includes a first channel forming section forming a first channel for allowing an oxygen-containing exhaust gas to flow in a horizontal direction, and a second channel forming section forming a second channel connected to the first channel. The first channel forming section is provided with an inlet for guiding the oxygen-containing exhaust gas into the first channel. The second channel forming section is provided with an outlet for discharging the oxygen-containing exhaust gas flowing through the second channel. The second channel includes a bent channel for guiding upward the oxygen-containing exhaust gas guided from the first channel.