A method of constructing an ink cartridge is disclosed. The method comprises the steps of: providing an ink bag with an ink supply port at one end and an ink charge opening at an opposite end thereof; then affixing a back member to an external flat side of the ink bag; then suspending the ink bag with the ink supply port facing downward and charging a predetermined amount of ink into the ink bag via the ink charge opening; then permanently sealing the ink charge opening; keeping the ink bag flat with the back member; and affixing the back member to the bottom of a case main body.