A gas-liquid separator for a fuel cell system onboard a vehicle includes an upper chamber, a lower chamber, a plate for separating the upper and lower chambers, a pipe for circulating the exhaust gas and connecting holes bored in the plate. The upper chamber receives exhaust gas from the fuel cell system to separate water contained in the exhaust gas. The lower chamber has a portion for discharging the water which is separated by the upper chamber. The pipe, which is attached to the upper chamber, has fluid communication with an inside of the upper chamber. The connecting holes provide fluid communication between the upper and lower chambers. The connecting holes are positioned off a center of the plate so that the connecting holes lie apart from the pipe.