A gas-liquid separator includes an inlet pipe through which a gas-liquid two-phase fluid flows and a swirling flow generating ribbon disposed within the inlet pipe to swirl the gas-liquid two-phase fluid along an inner surface of the inlet pipe, wherein the inner surface of the inlet pipe includes a first step surface at a location downstream of a flow direction of the gas-liquid two-phase fluid from the swirling flow generating ribbon, the first step surface increasing an inner diameter of the inlet pipe downward thereof.