Flow averaging level control (FALC) is employed in several chemical processes so that fluctuations in outflow from the process can be attenuated against disturbances as well as load changes in inflow to the process while the level is kept around the setpoint. In this paper, a practical scheme of designing the FALC is proposed. Assuming that the process has an integral element, the structure of a level control system becomes a second order system, which can be specified by the time constant as well as damping constant. The proposed scheme determines these two values so as to optimize two performance indexes : indexes of level controllability and attenuability of fluctuation in the outflow. The controllability index is defined by the integral of squared error of the level control and that of attenuability is defined by the integral of squared derivative of the manipulated variable, outflow rate. The damping constant for optimizing performance is derived and it is 1/root 2 (Relative Damping). The magnitudes of disturbance as well as allowable ranges of level fluctuation in both nominal and load change operations are given as the design specifications. By giving specifications to the controller, an IP controller with a gap function is designed so as to maintain the optimal damping constant of the control system. The proposed scheme is applied to industrial plants to illustrate the validity of the scheme.