An automatic gain controller having an estimator to estimate the level of an input signal, a subtractor to calculate a difference between the level estimated by the estimator and a predetermined reference value, a gain controller to output a control value achieved by scaling non-linearly the difference output by the subtractor, an accumulator to accumulate the output of the gain controller, and a modulator to modulate the output of the accumulator to a pulse width modulated signal. The gain controller outputs the control value proportional to a square of the difference. If en error between the power of the input signal and the reference value is great, the power can be quickly adjusted to a desired signal level, and if the error is small, the gain can be controlled finely. |