An ice making system for a refrigerator, the ice making system comprising: an ice making unit that makes ice cubes; a cold air generator that cools air inside a cooling duct so as to produce cold air; a cold air circulation unit that supplies the cold air from the cold air generator to the ice making unit and discharges the cold air from the ice making unit to the cold air generator; an opening/closing unit that discharges defrost water produced from the cooling duct to an outside; and a drainage unit that drains the defrost water discharged from the opening/closing unit to the outside through a drain hose communicating with the opening/closing unit, the cold air generator comprises: the cooling duct through which the air flows; an evaporation coil such that the air is cooled by a heat exchanging operation between the air and a refrigerant; a compressor that compresses the refrigerant discharged from the evaporation coil so as to change the refrigerant to a high temperature and high pressure gas refrigerant; a condenser that condenses the gas refrigerant so as to change the gas refrigerant to a high pressure liquid refrigerant; an expansion valve that performs adiabatic expansion of the liquid refrigerant and supplies the refrigerant to the evaporation coil; and a heater that defrosts the cooling duct by heating the cooling duct, thereby producing the defrost water; the ice making system being characterized in that: the opening/closing unit comprises: a cap provided in a lowermost part of the cooling duct such that the cap communicates with the cooling duct, the cap being combined with an upper end of the drain hose; and a lid connected to a predetermined portion of the cap such that the lid opens the cap in response to a weight of the defrost water, and wherein the evaporation coil is wound around the cooling duct. an ice making unit that makes ice cubes; a cold air generator that cools air inside a cooling duct so as to produce cold air; a cold air circulation unit that supplies the cold air from the cold air generator to the ice making unit and discharges the cold air from the ice making unit to the cold air generator; an opening/closing unit that discharges defrost water produced from the cooling duct to an outside; and a drainage unit that drains the defrost water discharged from the opening/closing unit to the outside through a drain hose communicating with the opening/closing unit, the cold air generator comprises: the cooling duct through which the air flows; an evaporation coil such that the air is cooled by a heat exchanging operation between the air and a refrigerant; a compressor that compresses the refrigerant discharged from the evaporation coil so as to change the refrigerant to a high temperature and high pressure gas refrigerant; a condenser that condenses the gas refrigerant so as to change the gas refrigerant to a high pressure liquid refrigerant; an expansion valve that performs adiabatic expansion of the liquid refrigerant and supplies the refrigerant to the evaporation coil; and a heater that defrosts the cooling duct by heating the cooling duct, thereby producing the defrost water; the cooling duct through which the air flows; an evaporation coil such that the air is cooled by a heat exchanging operation between the air and a refrigerant; a compressor that compresses the refrigerant discharged from the evaporation coil so as to change the refrigerant to a high temperature and high pressure gas refrigerant; a condenser that condenses the gas refrigerant so as to change the gas refrigerant to a high pressure liquid refrigerant; an expansion valve that performs adiabatic expansion of the liquid refrigerant and supplies the refrigerant to the evaporation coil; and a heater that defrosts the cooling duct by heating the cooling duct, thereby producing the defrost water; the ice making system being characterized in that: the opening/closing unit comprises: a cap provided in a lowermost part of the cooling duct such that the cap communicates with the cooling duct, the cap being combined with an upper end of the drain hose; and a lid connected to a predetermined portion of the cap such that the lid opens the cap in response to a weight of the defrost water, and wherein the evaporation coil is wound around the cooling duct. a cap provided in a lowermost part of the cooling duct such that the cap communicates with the cooling duct, the cap being combined with an upper end of the drain hose; and a lid connected to a predetermined portion of the cap such that the lid opens the cap in response to a weight of the defrost water, and wherein the evaporation coil is wound around the cooling duct.