An ink cartridge 1 constructed as a liquid container has wall faces 1a, 370w1, and 370w2 that are respectively pierced to have holes. One end of an ink supply tube 910 is inserted through the holes of the wall faces 1a, 370w1, and 370w2 and is connected with an inlet 401 of a vertical communicating path 400 located in the upstream of a sensor unit 30 functioning as a detector. The other end of the ink supply tube 910 is connected to a large-capacity ink tank 900. Attachment of the ink cartridge 1 to an ink-jet printer completes an ink supply system. This arrangement effectively controls or prevents migration of bubbles into the detector in the liquid container equipped with the detector.