Abstract: A projectable or push-out type writing instrument cartridge, such as a projectable cartridge for a ball point pen, has a filler impregnated with an aqueous ink, a writing point at its one end, a device for feeding the ink to the writing point, and an end plug mounted in the other end of the cartridge. The plug has an air passage which satisfies a predetermined condition. Preferably, the end plug consists of an inner rod and an outer tube which receives the inner rod in such a manner that the air passage is formed between the rod and the tube. The cartridge is secured in the casing of the instrument, and is projectable to a writing position.