A washing machine comprising: a cabinet; a tub disposed in the cabinet to receive water; a drum rotatably disposed in the tub to contain a laundry; and a detergent supply device disposed at the cabinet to supply detergent into the tub; wherein the detergent supply device comprises: a plurality of cartridges containing detergent; a pump for suctioning detergent from the plurality of cartridges; a passage (700, 800) guiding detergent discharged from the pump into the tub; and a plurality of electrode sensors (301a, 301b, 301c) disposed at a side of the plurality of cartridges; wherein each of the plurality of electrode sensors (301a, 301b, 301c) comprises: an electrode plate (321a, 321b, 321c) configured for detecting electric signal; and a terminal (311a, 311b, 311c) for transmitting the electric signal detected by the electrode plate, wherein the electrode plate (321a, 321b, 321c) has a first surface contacted with detergent contained in the cartridge and a second surface connected to the terminal, characterized in that the electrode plate (321a, 321b, 321c) is disposed to have, in its upright position, a smaller width at a lower end thereof than at an upper end thereof, wherein an opening (216a, 216b, 216c) is formed at a rear wall of the cartridge, and the electrode plate (321a, 321b, 321c) is disposed to be in contact with the rear wall of the cartridge so that the first surface thereof is exposed inside the cartridge through the opening, and wherein the opening (216a, 216b, 216c) is in contact with the lower end of the electrode plate. a cabinet; a tub disposed in the cabinet to receive water; a drum rotatably disposed in the tub to contain a laundry; and a detergent supply device disposed at the cabinet to supply detergent into the tub; wherein the detergent supply device comprises: a plurality of cartridges containing detergent; a pump for suctioning detergent from the plurality of cartridges; a passage (700, 800) guiding detergent discharged from the pump into the tub; and a plurality of electrode sensors (301a, 301b, 301c) disposed at a side of the plurality of cartridges; a plurality of cartridges containing detergent; a pump for suctioning detergent from the plurality of cartridges; a passage (700, 800) guiding detergent discharged from the pump into the tub; and a plurality of electrode sensors (301a, 301b, 301c) disposed at a side of the plurality of cartridges; wherein each of the plurality of electrode sensors (301a, 301b, 301c) comprises: an electrode plate (321a, 321b, 321c) configured for detecting electric signal; and a terminal (311a, 311b, 311c) for transmitting the electric signal detected by the electrode plate, wherein the electrode plate (321a, 321b, 321c) has a first surface contacted with detergent contained in the cartridge and a second surface connected to the terminal, characterized in that the electrode plate (321a, 321b, 321c) is disposed to have, in its upright position, a smaller width at a lower end thereof than at an upper end thereof, wherein an opening (216a, 216b, 216c) is formed at a rear wall of the cartridge, and the electrode plate (321a, 321b, 321c) is disposed to be in contact with the rear wall of the cartridge so that the first surface thereof is exposed inside the cartridge through the opening, and wherein the opening (216a, 216b, 216c) is in contact with the lower end of the electrode plate. an electrode plate (321a, 321b, 321c) configured for detecting electric signal; and a terminal (311a, 311b, 311c) for transmitting the electric signal detected by the electrode plate, wherein the electrode plate (321a, 321b, 321c) has a first surface contacted with detergent contained in the cartridge and a second surface connected to the terminal, characterized in that the electrode plate (321a, 321b, 321c) is disposed to have, in its upright position, a smaller width at a lower end thereof than at an upper end thereof, wherein an opening (216a, 216b, 216c) is formed at a rear wall of the cartridge, and the electrode plate (321a, 321b, 321c) is disposed to be in contact with the rear wall of the cartridge so that the first surface thereof is exposed inside the cartridge through the opening, and wherein the opening (216a, 216b, 216c) is in contact with the lower end of the electrode plate.