A discharge flow rate of a loss-in-weight scale is controlled by continuously changing control outputs to save the time necessary for data stabilization. A curve relating the discharge flow rate and the control output is obtained by using the functional relationship between the control output and time and the functional relationship between the material weight and time. The discharge flow rate can be precisely controlled by directly selecting or inputting a value of the control output, to achieve the objectives of direct controlling and of avoiding the time for data stabilization.