An ink cartridge has a first partitioning wall 5 and a third partitioning wall 7 for dividing the inside of a case 2 into a first ink chamber 9 and a second ink chamber 10, and a second partitioning wall 6 for demarcating an atmosphere connection chamber 11. In an upper case cover 3 are formed an ink filling hole 13 communicating with the second ink chamber 10 and a pressure reduction hole 14 communicating with the atmosphere connection chamber 11. Therefore, when ink filling is being done, the filling apparatus and the pressure reduction apparatus need only be brought to bear from one side of the upper case cover, resulting in improved work efficiency. Ink filling can be performed efficiently because the ink successively passes to the second ink chamber 10 and the first ink chamber 9. After ink filling, the ink filling hole 13 and the pressure reduction hole 14 can easily be sealed off with sealing material. An ink cartridge is thus provided that can be easily assembled at low cost.