A liquid container which can precisely detects existence/absence of residual liquid at a low cost, a cartridge including the liquid container, and a printing apparatus using the cartridge. The lengthwise direction of a triangular prism provided on the bottom of the ink tank is along a direction of print medium conveyance in the printing apparatus, such that when an ink-jet head cartridge holding the ink tank is attached to the printing apparatus for performing printing, light emitted from an optical unit of the printing apparatus is precisely captured by the prism. Further, if two ink-jet head cartridges are mounted on the printing apparatus, the first and second ink-jet head cartridges are set in positions shifted from each other by a length shorter than the length in the lengthwise direction of the prism, such that the light emitted from the optical unit reaches the prisms provided in the ink tanks of the first and second ink-jet head cartridges.