Abstract: A liquid measuring apparatus is designed for use in liquid chromatographic separation. The apparatus includes a plug body to be connected to the mouth or opening of the liquid container for opening/closing the liquid container, a sound wave producer disposed inside the plug body for providing output of sound wave directed toward the liquid, a receiver disposed inside the plug body for receiving a standing wave composed of an original sound wave provided by the sound wave producer and that part of the original sound wave which has hit against the surface of the liquid and has been reflected by the surface of the liquid, an algorithm for calculating the resonant frequency of the standing wave received by the receiver, and an algorithm for computing the amount of the liquid that remains in the liquid container.