1. An actuator control apparatus comprising: an observation unit configured to estimate a state of a load based on an equation including at least one modeled load parameter; a controller which outputs a signal for controlling the load; a compensation unit configured to compensate for the signal output from the controller; and an estimator configured to estimate a change of a load parameter, to decide a gain of the compensation unit based on the estimated change of the load parameter, and to update the modeled load parameter.