<p>A plant control system has a reference value setting unit (28) for variably setting a reference value for an air-fuel ratio to be given to an exhaust system (E) including a catalytic converter (3), depending on a component based on an adaptive control law of a manipulated variable of the air-fuel ratio generated by a controller according to an adaptive sliding mode control process in order to converge an output of an 02 sensor (6) disposed downstream of the catalytic converter to a target value. The manipulated variable generated by the controller represents the difference between the air-fuel ratio and the reference value, required to converge the output of the 02 sensor to the target value. <IMAGE></p>
