An ice making system for a refrigerator, the ice making system comprising: an ice making unit configured to make ice cubes; a cold air generator, which includes a cooling duct through which air flows, configured to cool the air inside the cooling duct so as to produce cold air; a cold air circulation unit configured to supply 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; and a drainage unit configured to drain defrost water produced from the cooling duct to the outside, wherein the drainage unit comprises a hollow drain hose through which the defrost water flows for draining, the drain hose being connected to a lowermost bent part of the cooling duct, wherein the ice making unit is configured to be provided on a refrigeration compartment door of the refrigerator, and wherein the cold air generator further includes an evaporation coil through which a refrigerant flows, characterized in that - the evaporation coil is wound around the cooling duct to cool the air flowing through the cooling duct by a heat exchanging operation between the air and the refrigerant,. - the cooling duct has a U-shape and is configured to be provided in a sidewall of a refrigeration compartment of a refrigerator body, - the U-shaped cooling duct includes a first duct hole at one end thereof and a second duct hole at the other end thereof, - the cold air circulation unit includes an inlet hole provided on an upper part of the ice making unit to be connected to the first duct hole and an outlet hole provided on a lower part of the ice making unit to be connected to the second duct hole, so that the cooling duct communicates with the ice making unit when the refrigeration compartment door is closed. an ice making unit configured to make ice cubes; a cold air generator, which includes a cooling duct through which air flows, configured to cool the air inside the cooling duct so as to produce cold air; a cold air circulation unit configured to supply 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; and a drainage unit configured to drain defrost water produced from the cooling duct to the outside, wherein the drainage unit comprises a hollow drain hose through which the defrost water flows for draining, the drain hose being connected to a lowermost bent part of the cooling duct, wherein the ice making unit is configured to be provided on a refrigeration compartment door of the refrigerator, and wherein the cold air generator further includes an evaporation coil through which a refrigerant flows, - the evaporation coil is wound around the cooling duct to cool the air flowing through the cooling duct by a heat exchanging operation between the air and the refrigerant,. - the cooling duct has a U-shape and is configured to be provided in a sidewall of a refrigeration compartment of a refrigerator body, - the U-shaped cooling duct includes a first duct hole at one end thereof and a second duct hole at the other end thereof, - the cold air circulation unit includes an inlet hole provided on an upper part of the ice making unit to be connected to the first duct hole and an outlet hole provided on a lower part of the ice making unit to be connected to the second duct hole, so that the cooling duct communicates with the ice making unit when the refrigeration compartment door is closed.