An ink tank, which is mountable on an ink jet printer, includes an ink outlet adapted to supply ink stored in the ink tank to the ink jet printer through the outlet; an air communicating hole configured to introduce air into the ink tank through the hole in accordance with an amount of the ink supplied to the ink jet printer from the ink tank; a passage communicating with the ink outlet; a prism formed in the ink tank and adapted to detect whether a remaining amount of the ink in the ink tank is below a predetermined level or not; a filter disposed between the prism and the passage; and an ink chamber connected to an upstream opening of the passage. The prism and the filter are disposed in the ink chamber.