A refrigerator comprising: a main body provided with storage chambers (111F, 111R); doors to open and close the storage chambers (111F, 111R); an opening provided on any one of the doors; a sub-door installed at the opening to open and close the opening, wherein a lower end of the sub-door is hinged to a lower end of the opening such that the sub-door is vertically rotatable to open and close the opening; a sub-door support part which is provided on the door and protrudes inwards toward the opening to support a rear surface of the sub-door; a locking member formed in a rod shape which is provided on the sub-door; and a locking device which is adapted to lock the locking member to maintain a state in which the opening is closed by the sub-door, wherein: the opening comprises a first opening region (20c-1) and a second opening region (20c-2), the first opening region (20c-1) is configured to receive the sub-door therein and being adjacent to the second opening region (20c-2), the second opening region (20c-2) is defined by the sub-door support part; characterized in that at least a part of the locking device is embedded in an upwardly depressed locking device mount recess of the door which is formed adjacent to an upper portion of the first opening region (20c-1), wherein the locking device includes a rotary hook rotatable in rightward and leftward directions to be selectively locked with the locking member, and wherein the locking member protrudes upward from an upper portion of an inner surface of the sub-door when the sub-door is in a closed position so as to be coupled to the locking device, when the sub-door is in the closed position. a main body provided with storage chambers (111F, 111R); doors to open and close the storage chambers (111F, 111R); an opening provided on any one of the doors; a sub-door installed at the opening to open and close the opening, wherein a lower end of the sub-door is hinged to a lower end of the opening such that the sub-door is vertically rotatable to open and close the opening; a sub-door support part which is provided on the door and protrudes inwards toward the opening to support a rear surface of the sub-door; a locking member formed in a rod shape which is provided on the sub-door; and a locking device which is adapted to lock the locking member to maintain a state in which the opening is closed by the sub-door, wherein: the opening comprises a first opening region (20c-1) and a second opening region (20c-2), the first opening region (20c-1) is configured to receive the sub-door therein and being adjacent to the second opening region (20c-2), the second opening region (20c-2) is defined by the sub-door support part; characterized in that at least a part of the locking device is embedded in an upwardly depressed locking device mount recess of the door which is formed adjacent to an upper portion of the first opening region (20c-1), wherein the locking device includes a rotary hook rotatable in rightward and leftward directions to be selectively locked with the locking member, and wherein the locking member protrudes upward from an upper portion of an inner surface of the sub-door when the sub-door is in a closed position so as to be coupled to the locking device, when the sub-door is in the closed position. the opening comprises a first opening region (20c-1) and a second opening region (20c-2), the first opening region (20c-1) is configured to receive the sub-door therein and being adjacent to the second opening region (20c-2), the second opening region (20c-2) is defined by the sub-door support part; characterized in that at least a part of the locking device is embedded in an upwardly depressed locking device mount recess of the door which is formed adjacent to an upper portion of the first opening region (20c-1), wherein the locking device includes a rotary hook rotatable in rightward and leftward directions to be selectively locked with the locking member, and wherein the locking member protrudes upward from an upper portion of an inner surface of the sub-door when the sub-door is in a closed position so as to be coupled to the locking device, when the sub-door is in the closed position.