An apparatus for measuring the amount of ink remaining in an ink tank using a sensing module and a detecting circuit is disclosed. The sensing module comprises a fixed device connected to the casing of the ink tank and a movable device connected to the bag. As the ink is ejected, the volume of the bag increases so that the movable device moves along with the bag. The detecting circuit, coupled with the sensing module, generates a corresponding detecting signal according to the variation in the relative position of the movable device and the fixed device, and the measurement of the ink amount is thus obtained.