Operation control method employable in a control apparatus controls a control object by calculating an 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 the 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.