An effect adding method, includes: applying different gains to a positive side waveform portion and a negative side waveform portion of an audio signal respectively when absolute values of input levels of the positive side waveform portion and the negative side waveform portion are smaller than a predetermined value; producing a higher range component of the audio signal based on a high range component of the audio signal to which the gain is applied, the higher range component being higher in frequency than the high range component; producing a lower range component of the audio signal based on a low range component of the audio signal to which the gain is applied, the lower range component being lower in the frequency than the low range component; and synthesizing an audio signal having an effect sound by adding the audio signal to which the different gains are applied, the higher range component, and the lower range component with each other.