To fill the ink cartridge with the ink, a first air introduction hole of a negative pressure generator chamber is sealed up, and a pressure pump is connected to a second air introduction hole of a storage chamber, to raise pressure inside the storage chamber so as to exhaust air out of an ink bag. While keeping the storage chamber under the high pressure, the second air introduction hole is sealed up, the first air introduction hole is opened, and an ink filling pump is activated to inject the ink through an ink outlet into the negative pressure generator chamber. When a predetermined amount of ink is injected, the second air introduction hole is opened, and the ink filling pump is reactivated to fill up the ink bag as well as the negative pressure generator chamber with the ink.