To provide a liquid quantity management system capable of performing highly accurate calibration processing.

When calibrating a conversion table for specifying a relation between the liquid level and the liquid quantity in a liquid storage tank 1, metering data is corrected by instrumental error data of a metering machine 3, and a liquid supply quantity (ΔQ) supplied to the metering machine 3 is determined, and a liquid quantity gradient (ΔQ/ΔH) is calculated from the liquid supply quantity and a liquid level change (ΔH) before and after liquid supply detected by a liquid level detection machine 2, and the calibration processing is performed by using data of the liquid quantity gradient. When calculating the liquid quantity after calibration from the liquid quantity gradient, the liquid level corresponding to a half of the whole capacity of the liquid storage tank 1 is determined, and the liquid quantity at the liquid level is set as a half of the whole capacity of the liquid storage tank 1, and the liquid quantity after calibration at each liquid level is calculated based thereon.