A hydrogen purging device for a fuel cell system includes a humidifier that humidifies dry air supplied from an air blower, using moist air discharged from a cathode of a stack and supplies the humidified air to the cathode. A water trap and a hydrogen recirculation blower are sequentially connected to an outlet of an anode, wherein a hydrogen outlet of the water trap and an inlet of the humidifier are connected by a cathode-hydrogen purging line for purging hydrogen to the cathode so that the hydrogen discharged from the anode of the fuel stack is purged to the cathode during idling or during normal driving.