A method of filling an ink cartridge which has (i) an ink supplying portion, (ii) an air introducing portion, (ii) a first valve member disposed in the ink supplying portion and biased in a biased direction that causes the first valve member to be placed in its closed position, and (iv) a second valve member disposed in the air introducing portion and biased in a biased direction that causes the second valve member to be placed in its closed position. The method including: (a) moving each of the first and second valve members in the opposite direction away from the closed position; and (b) supplying the ink into the ink cartridge through one of the air introducing portion and the ink supplying portion, while discharging the air out of the ink cartridge through the other of the air introducing portion and the ink supplying portion. Also disclosed are a cartridge filler, a jig and an ink supply system.