A control system is provided for controlling a process system subject to an external disturbance by adjusting the value of a control amount output from the process system to a given target value. The system includes a target value generating unit for generating a given target value, and a main control unit receiving the target value and the control amount value for performing control operations to suppress fluctuation of the control amount value produced by the external disturbance and to adjust the control amount value to the given target value. The main control unit includes a suppressing unit for suppressing fluctuation of the control amount value produced by the external disturbance, an adjusting unit for adjusting a proportional gain of the suppressing unit, derivative time of the suppressing unit, and the control amount value to the given target value, a proportional gain revision coefficient unit for setting a plurality of given proportional gain revision coefficients, and a coefficient selecting unit for selecting the given proportional gain revision coefficient from among the plurality of given proportional gain revision coefficients.