In an ink jet recording apparatus of the ink-on demand type, the recording ink is forcibly sent from a pressure chamber in the recording head to nozzles which jet the recording ink onto a recording medium. An ink cartridge is a closed-up rigid body having an opening therein which permits gas to flow in and out of the rigid body. The rigid cartridge body contains a flexible ink container bag which contains recording ink. A pressurized gas supply is connected to the ink cartridge body in communication with the opening thereof for increasing the pressure inside the ink cartridge body and for thereby increasing the pressure of the ink in the flexible container bag due to flexing of the bag under increased pressure. A cap made of elastic material is coupled to the cartridge body so as to be connected to the ink container bag in a liquid-type manner to supply recording ink from inside the ink container bag to the recording head under pressure to purge the recording head of bubbles and of ink previously contained therein.