An ink cartridge for an ink jet printer is provided with a container having at least a first wall. An ink supply port is formed through the first wall for supplying ink to the exterior of the container. An ink chamber and an ink supply chamber are formed by a membrane valve seat made of an elastic thin membrane and formed with a through hole therethrough. A valve body is positioned opposing the through hole formed in the membrane valve seat, the valve body and membrane valve seat adjacent said through hole formed therein being maintained selectively in contact by a pressure difference between said ink chamber and said ink supply chamber.