A threshold amplitude is calculated for each subband obtained by splitting an extension band. For each subband, an amplitude of transform coefficients is compared with the threshold amplitude to extract a transform coefficient having an amplitude larger than the threshold amplitude as a representative transform coefficient. When a number of the extracted representative transform coefficients is less than a predetermined number, the threshold amplitude is updated in accordance with an amount by which the number of the representative transform coefficients is less than the predetermined number. A transform coefficient is extracted again using the updated threshold amplitude. For each of the subbands, a value of correlation is calculated between the representative transform coefficient and a normalized core encoded low-band transform coefficient. A subband having a largest value of correlation is selected when the number of the extracted representative transform coefficients reaches the predetermined number.