Apparatus, method, and medium for processing an audio signal using a correlation between bands are provided. The apparatus includes an encoding unit encoding an input audio signal and a decoding unit decoding the encoded input audio signal. The encoding unit includes a correlation analyzer searching a most subband having a correlation of more than a predetermined value between a first subband and the most similar subband in each of the first subbands from second subbands and generating information about the second searched subband, and the decoding unit comprises a high frequency component restoring portion copying data about the second searched subband as data about the first subband, using the generated information about the second subband generated by the correlation analyzer and transmitted in a bit stream format, to perform decoding on the first subbands, and the first subbands are subbands that belong to a high frequency band in a band of a result of subband-filtering the input audio signal and the second subbands are subbands that belong to a low frequency band in a band of the result of subband-filtering.