An improved apparatus for controlling a mass flow rate of liquid in a chemical process, wherein a control system for regulating a mass flow rate of liquid in a liquid passage of the process is automatically controlled in accordance with a deviation between a reference input of mass flow rate of liquid, and an average mass flow rate of liquid measured by a particularly designed measuring device. The measuring device is provided with a liquid container which is disposed in a liquid passage of the process and supported by a weighing mechanism. An opening or closing means and an automatic means for regulating the mass flow rate of liquid are mounted separately on the liquid passage at both opposed sides of the liquid container respectively, and the automatic regulating means is actuated as mentioned above. After opening or closing the opening or closing means, a weight difference Delta W of liquid contained in the liquid container is measured at two measuring instances which are separated by a time interval Delta T, then the average mass flow rate of the liquid is integrally calculated by an equation ( Delta W/ Delta T). The measuring of the liquid weight is carried out together with measuring the net weight of the liquid container by way of the weighing mechanism.