A first differential value is acquired between first current data and first previous data in an i number (i being a natural number) of sampling periods before the current data. A second differential value is acquired between second current data and second previous data in a j number (j being a natural number) of sampling periods before the current data. Both first data and both second data are of a first and a second digital audio signal, respectively, having a sound level of a digital stereo audio signal in the left and right channels, respectively. A first and a second correction coefficient are acquired by adding the first and second differential values at a first and a second ratio, respectively. The first signal is corrected by multiplying the first signal by the first correction coefficient. The second signal is corrected by multiplying the second signal by the second correction coefficient.