A refrigerator comprising: a main body defining a storage space; a compartment provided in the storage space, where a temperature in the compartment is lower than -20 degrees Celsius; and a thermoelectric module assembly disposed at one side of the compartment so that the compartment is cooled to a temperature less than that of the storage space, wherein the compartment comprises: a case into which an insulation material is filled to be thermally insulated from the storage space and in which a space is defined; a case door configured to open and to close the case; and characterized by a rail assembly connecting the case to the case door and extending and contracted in multi-stages to allow the case door to be slid to be inserted and withdrawn, wherein the rail assembly is mounted on the case outside the space, the refrigerator further comprises a rail mounting part to which the rail assembly is fixed and mounted, and which is disposed on a bottom surface of the case; and a rail cover fixed to a rear surface of the case door and extending along the rail assembly to cover the rail assembly, wherein a cover guide part accommodating the rail cover when the case door is inserted and withdrawn is provided on the bottom surface of the case. a main body defining a storage space; a compartment provided in the storage space, where a temperature in the compartment is lower than -20 degrees Celsius; and a thermoelectric module assembly disposed at one side of the compartment so that the compartment is cooled to a temperature less than that of the storage space, wherein the compartment comprises: a case into which an insulation material is filled to be thermally insulated from the storage space and in which a space is defined; a case door configured to open and to close the case; and characterized by a rail assembly connecting the case to the case door and extending and contracted in multi-stages to allow the case door to be slid to be inserted and withdrawn, wherein the rail assembly is mounted on the case outside the space, the refrigerator further comprises a rail mounting part to which the rail assembly is fixed and mounted, and which is disposed on a bottom surface of the case; and a rail cover fixed to a rear surface of the case door and extending along the rail assembly to cover the rail assembly, wherein a cover guide part accommodating the rail cover when the case door is inserted and withdrawn is provided on the bottom surface of the case. a case into which an insulation material is filled to be thermally insulated from the storage space and in which a space is defined; a case door configured to open and to close the case; and characterized by a rail assembly connecting the case to the case door and extending and contracted in multi-stages to allow the case door to be slid to be inserted and withdrawn, wherein the rail assembly is mounted on the case outside the space, the refrigerator further comprises a rail mounting part to which the rail assembly is fixed and mounted, and which is disposed on a bottom surface of the case; and a rail cover fixed to a rear surface of the case door and extending along the rail assembly to cover the rail assembly, wherein a cover guide part accommodating the rail cover when the case door is inserted and withdrawn is provided on the bottom surface of the case.