A method includes: acquiring time-series data of a control input and a control output; calculating a variable parameter that minimizes an evaluation function, based on observation values of the control input and the control output; and updating the variable parameter with the calculated variable parameter. The evaluation function includes: a first function part that evaluates an entire error that is a control error of the entire observation period and increases the output value as the entire error increases; and at least one of: a second function part that evaluates a sectional error that is a control error of a particular section in the observation period and increases the output value as the sectional error increases; and a third function part that evaluates an instantaneous error that is a control error of a particular time point in the observation period and increases the output value as the instantaneous error increases.