A discharge port is located at a lower portion of the case of a gas-liquid separator. A discharge valve is located at the discharge port. A water retaining portion is located at the bottom of the case. The water retaining portion is located at a position lower than the discharge valve. An upward inclination surface is formed on the bottom of the water retaining portion. The upward inclination surface is inclined upward toward the discharge valve. A downward inclination surface is formed on the bottom of the water retaining portion. The downward inclination surface is inclined downward toward the upward inclination surface. A cover portion is located in an upper portion of the water retaining portion. The cover portion defines a gas passage in an upper portion of the water retaining portion. The gas passage is open at a portion closer to the inlet and connected to the discharge valve.