A washing machine comprising: a cabinet including a first opening; a tub positioned inside the cabinet, and including a second opening and a protruding rib protruding toward a center of the second opening; a drum rotatably positioned inside the tub, and including a third opening; and a diaphragm connecting the first opening with the second opening, and including a first flange and a second flange that are coupled to the tub, wherein the first flange is fixed to an outer surface of the tub and the second flange is coupled to the protruding rib to maintain a uniform gap between the diaphragm and the third opening around an outer circumference of the second opening wherein the second flange comprises: a first extension portion contacting one surface of the protruding rib, a second extension portion contacting another surface of the protruding rib, and a third extension portion contacting the other surface of the protruding rib and forming a protruding rib accommodating space accommodating the protruding rib, wherein the tub further comprises: a tub body; and a tub lip portion extending from the tub body to couple to the diaphragm, and defining the outer circumference of the second opening, wherein the tub lip portion comprises: a first lip portion coupled to the first flange; and a second lip portion coupled to the second flange, and including the protruding rib and a resting rib, and characterized in that the resting rib extends toward the inside of the tub from the protruding rib in a perpendicular direction from the resting rib. a cabinet including a first opening; a tub positioned inside the cabinet, and including a second opening and a protruding rib protruding toward a center of the second opening; a drum rotatably positioned inside the tub, and including a third opening; and a diaphragm connecting the first opening with the second opening, and including a first flange and a second flange that are coupled to the tub, wherein the first flange is fixed to an outer surface of the tub and the second flange is coupled to the protruding rib to maintain a uniform gap between the diaphragm and the third opening around an outer circumference of the second opening wherein the second flange comprises: a first extension portion contacting one surface of the protruding rib, a second extension portion contacting another surface of the protruding rib, and a third extension portion contacting the other surface of the protruding rib and forming a protruding rib accommodating space accommodating the protruding rib, wherein the tub further comprises: a tub body; and a tub lip portion extending from the tub body to couple to the diaphragm, and defining the outer circumference of the second opening, wherein the tub lip portion comprises: a first lip portion coupled to the first flange; and a second lip portion coupled to the second flange, and including the protruding rib and a resting rib, and characterized in that the resting rib extends toward the inside of the tub from the protruding rib in a perpendicular direction from the resting rib. a first extension portion contacting one surface of the protruding rib, a second extension portion contacting another surface of the protruding rib, and a third extension portion contacting the other surface of the protruding rib and forming a protruding rib accommodating space accommodating the protruding rib, wherein the tub further comprises: a tub body; and a tub lip portion extending from the tub body to couple to the diaphragm, and defining the outer circumference of the second opening, wherein the tub lip portion comprises: a first lip portion coupled to the first flange; and a second lip portion coupled to the second flange, and including the protruding rib and a resting rib, and characterized in that the resting rib extends toward the inside of the tub from the protruding rib in a perpendicular direction from the resting rib. a tub body; and a tub lip portion extending from the tub body to couple to the diaphragm, and defining the outer circumference of the second opening, wherein the tub lip portion comprises: a first lip portion coupled to the first flange; and a second lip portion coupled to the second flange, and including the protruding rib and a resting rib, and characterized in that the resting rib extends toward the inside of the tub from the protruding rib in a perpendicular direction from the resting rib. a first lip portion coupled to the first flange; and a second lip portion coupled to the second flange, and including the protruding rib and a resting rib, and characterized in that the resting rib extends toward the inside of the tub from the protruding rib in a perpendicular direction from the resting rib.