A control system of the invention for regulating a quantity to be controlled based on a deviation of the controlled quantity from a target value thereof and control parameters includes a behavior feature value calculator, a basic state judgment data calculator and a control parameter setter. The behavior feature value calculator detects the period or frequency of behaviors of a specific kind performed by a subject to be controlled. The basic state judgment data calculator calculates the amount of variations in the aforementioned period or frequency. The control parameter setter updates the value of at least one of the control parameters based on the amount of the aforementioned variations.