A refrigerator comprising: a freezing chamber and a refrigerating chamber partitioned by a barrier; a heat exchange chamber disposed in the rear of the freezing chamber and receiving an evaporator for generating cooling air; a fan for discharging cooling air from the heat exchange chamber to the entire freezing chamber an ultra-low-temperature casing inserted into and withdrawn from the freezing chamber; the refrigerator further comprises a connecting unit directly connecting the heat exchange chamber to the ultra- low-temperature casing, wherein, the connecting unit comprises: a communication opening provided in the front of the evaporator, connecting the heat exchange chamber to the ultra-low-temperature casing, so that the ultra-low-temperature cooling air generated from the evaporator is directly discharged to the ultra-low-temperature storage; and characterised in that a blower fan is installed in the communication opening and directly blowing the ultra-low-temperature cooling air to the ultra-low-temperature casing through the communication opening, wherein, a cylindrical protrusion guide protruding forwards from the rear surface of an inner case of the freezing chamber and is formed in a periphery of the communication opening, and a recess is curved at the rear upper portion of the ultra-low-temperature casing with the same curvature as an outer diameter of the protrusion guide so that a portion of the periphery of the protrusion guide is seated thereon. a freezing chamber and a refrigerating chamber partitioned by a barrier; a heat exchange chamber disposed in the rear of the freezing chamber and receiving an evaporator for generating cooling air; a fan for discharging cooling air from the heat exchange chamber to the entire freezing chamber an ultra-low-temperature casing inserted into and withdrawn from the freezing chamber; the refrigerator further comprises a connecting unit directly connecting the heat exchange chamber to the ultra- low-temperature casing, wherein, the connecting unit comprises: a communication opening provided in the front of the evaporator, connecting the heat exchange chamber to the ultra-low-temperature casing, so that the ultra-low-temperature cooling air generated from the evaporator is directly discharged to the ultra-low-temperature storage; and characterised in that a blower fan is installed in the communication opening and directly blowing the ultra-low-temperature cooling air to the ultra-low-temperature casing through the communication opening, wherein, a cylindrical protrusion guide protruding forwards from the rear surface of an inner case of the freezing chamber and is formed in a periphery of the communication opening, and a recess is curved at the rear upper portion of the ultra-low-temperature casing with the same curvature as an outer diameter of the protrusion guide so that a portion of the periphery of the protrusion guide is seated thereon. a communication opening provided in the front of the evaporator, connecting the heat exchange chamber to the ultra-low-temperature casing, so that the ultra-low-temperature cooling air generated from the evaporator is directly discharged to the ultra-low-temperature storage; and characterised in that a blower fan is installed in the communication opening and directly blowing the ultra-low-temperature cooling air to the ultra-low-temperature casing through the communication opening, wherein, a cylindrical protrusion guide protruding forwards from the rear surface of an inner case of the freezing chamber and is formed in a periphery of the communication opening, and a recess is curved at the rear upper portion of the ultra-low-temperature casing with the same curvature as an outer diameter of the protrusion guide so that a portion of the periphery of the protrusion guide is seated thereon.