An aspect of this specification discloses a method of, based on a control input to a controlled object and on a control output from the controlled object, updating a value of a parameter in an estimator configured to estimate external force that acts on the controlled object. The method includes: determining whether the controlled object is in a particular state in which external force that acts on the controlled object is known; during a period in which it is determined that the controlled object is in the particular state, calculating, as a learning value, the value of the parameter that reduces an error between the known external force and estimated external force, the estimated external force being estimated by the estimator based on the control input and the control output;
and updating the value of the parameter in the estimator with the calculated learning value.