A refrigerator comprising: a cabinet defining a storage compartment; a main door having a front surface and a rear surface, the main door being rotatably connected to the cabinet and configured to open and close the storage compartment; an auxiliary storage compartment provided to the rear surface of the main door, an access opening being provided in the main door to allow access to the auxiliary storage compartment from the front surface of the main door; a sub-door having a front surface and a rear surface, the sub-door being rotatably connected to the main door and configured to open and close the access opening such that, based on the sub-door being oriented in a closed position, the rear surface of the sub-door contacts the front surface of the main door to close the access opening; and a button exposed at the front surface of the sub-door and configured to receive input provided by a user; a hook member protruding backward from the rear surface of the sub-door; a locking protrusion provided in a groove located on the front surface of the main door and configured to be selectively coupled to the hook member; characterized in that the hook member has a pivot shaft mounted to a rear end of the hook member and is configured to rotate within the sub-door about the pivot shaft in response to receipt of user input at the button; wherein the refrigerator further comprises: a link member configured to rotate the hook member in response to receipt of user input at the button; a first elastic member configured to return the hook member to an original position of the hook member; and a second elastic member configured to return the button to an original position of the button; wherein the second elastic member has a higher modulus of elasticity than that of the first elastic member such that the hook member returns to the original position thereof after the button returns to the original position thereof. a cabinet defining a storage compartment; a main door having a front surface and a rear surface, the main door being rotatably connected to the cabinet and configured to open and close the storage compartment; an auxiliary storage compartment provided to the rear surface of the main door, an access opening being provided in the main door to allow access to the auxiliary storage compartment from the front surface of the main door; a sub-door having a front surface and a rear surface, the sub-door being rotatably connected to the main door and configured to open and close the access opening such that, based on the sub-door being oriented in a closed position, the rear surface of the sub-door contacts the front surface of the main door to close the access opening; and a button exposed at the front surface of the sub-door and configured to receive input provided by a user; a hook member protruding backward from the rear surface of the sub-door; a locking protrusion provided in a groove located on the front surface of the main door and configured to be selectively coupled to the hook member; characterized in that the hook member has a pivot shaft mounted to a rear end of the hook member and is configured to rotate within the sub-door about the pivot shaft in response to receipt of user input at the button; wherein the refrigerator further comprises: a link member configured to rotate the hook member in response to receipt of user input at the button; a first elastic member configured to return the hook member to an original position of the hook member; and a second elastic member configured to return the button to an original position of the button; wherein the second elastic member has a higher modulus of elasticity than that of the first elastic member such that the hook member returns to the original position thereof after the button returns to the original position thereof. a link member configured to rotate the hook member in response to receipt of user input at the button; a first elastic member configured to return the hook member to an original position of the hook member; and a second elastic member configured to return the button to an original position of the button; wherein the second elastic member has a higher modulus of elasticity than that of the first elastic member such that the hook member returns to the original position thereof after the button returns to the original position thereof.