A washing machine comprising: a cabinet forming an exterior; a tub which is disposed in the cabinet and in which laundry water can be stored; a drum disposed inside the tub, whereby said drum is configured to load laundry therein and is rotatable with respect to the tub, a water supply unit for supplying the laundry water to the tub; a detergent box that can be supplied with the laundry water from the water supply unit so as to provide the laundry water to the tub; a first water supply tube for supplying the laundry water from the detergent box to the front of the tub; a second water supply tube for supplying the laundry water from the detergent box to either the side or rear of the tub; characterized by: a supply part disposed at the tub to guide the laundry water from the second water supply tube to an inside of the tub; a water storing part disposed inside the supply part, spaced apart from the supply part by a predetermined distance to form a first back-flow prevention path, and configured to store a part of water supplied from the second water supply tube; and a water guide installed inside the second water supply tube, configured to guide the laundry water from the second water supply tube to the tub side, a bottom end of which is inserted into an inside of the water storing part, and spaced apart from the water storing part by a predetermined distance to form a second back-flow prevention path, wherein the first back-flow prevention path is disposed to drop the laundry water to a space between the tub and the drum, wherein the laundry water supplied from the second water supply tube can be stored in the water storing part and can then pass through the second back-flow prevention path and the first back-flow prevention path to flow into the tub, wherein a lower side of the water storing part is disposed at a curved surface forming the tub, and wherein the lower side of the water storing part has a curved surface when viewed from a front of the washing machine. a cabinet forming an exterior; a tub which is disposed in the cabinet and in which laundry water can be stored; a drum disposed inside the tub, whereby said drum is configured to load laundry therein and is rotatable with respect to the tub, a water supply unit for supplying the laundry water to the tub; a detergent box that can be supplied with the laundry water from the water supply unit so as to provide the laundry water to the tub; a first water supply tube for supplying the laundry water from the detergent box to the front of the tub; a second water supply tube for supplying the laundry water from the detergent box to either the side or rear of the tub; characterized by: a supply part disposed at the tub to guide the laundry water from the second water supply tube to an inside of the tub; a water storing part disposed inside the supply part, spaced apart from the supply part by a predetermined distance to form a first back-flow prevention path, and configured to store a part of water supplied from the second water supply tube; and a water guide installed inside the second water supply tube, configured to guide the laundry water from the second water supply tube to the tub side, a bottom end of which is inserted into an inside of the water storing part, and spaced apart from the water storing part by a predetermined distance to form a second back-flow prevention path, wherein the first back-flow prevention path is disposed to drop the laundry water to a space between the tub and the drum, wherein the laundry water supplied from the second water supply tube can be stored in the water storing part and can then pass through the second back-flow prevention path and the first back-flow prevention path to flow into the tub, wherein a lower side of the water storing part is disposed at a curved surface forming the tub, and wherein the lower side of the water storing part has a curved surface when viewed from a front of the washing machine. a supply part disposed at the tub to guide the laundry water from the second water supply tube to an inside of the tub; a water storing part disposed inside the supply part, spaced apart from the supply part by a predetermined distance to form a first back-flow prevention path, and configured to store a part of water supplied from the second water supply tube; and a water guide installed inside the second water supply tube, configured to guide the laundry water from the second water supply tube to the tub side, a bottom end of which is inserted into an inside of the water storing part, and spaced apart from the water storing part by a predetermined distance to form a second back-flow prevention path, wherein the first back-flow prevention path is disposed to drop the laundry water to a space between the tub and the drum, wherein the laundry water supplied from the second water supply tube can be stored in the water storing part and can then pass through the second back-flow prevention path and the first back-flow prevention path to flow into the tub, wherein a lower side of the water storing part is disposed at a curved surface forming the tub, and wherein the lower side of the water storing part has a curved surface when viewed from a front of the washing machine.