It is an object to provide an operation control apparatus and an operation control method which allow operation of a control object without causing adverse affect on the operational condition of the control object, even when a deviation of a real system from a model (model error) arises. The operation control method employable in the control apparatus controls a control object by calculating operation amount to maximize or minimize an evaluation value obtained from a control deviation of the control object from a target value. The operation control method includes the steps of: establishing a model for simulating a property of the control object; calculating operation amount to maximize or minimize an evaluation value based on a control deviation of the model as a target; calculating an evaluation value based on a control deviation in controlling the control object by the operation amount; and determining an operation amount change width defined by a difference between current step operation amount and next step operation amount, based on the deviation of the control object from the model in the evaluation value of the control deviation.