An active noise control system includes a filter coefficient updating unit including an imaginary term estimator. The imaginary term estimator estimates an imaginary term Ie from a real term Re of an error signal e supplied from a microphone. The filter coefficient updating unit updates a filter coefficient W of an adaptive filter based on the imaginary term Ie, the real term Re, and a corrective signal supplied from a reference signal corrector. The filter coefficient updating unit updates the filter coefficient W successively in respective sampling periods.