Disclosed is an apparatus for extending a bandwidth of a sound signal. The apparatus includes a database that stores predetermined training information as a result of at least one of Gaussian mixture model (GMM) training and hidden Markov model (HMM) training; a modified discrete cosine transform (MDCT) transformer that transforms a first band signal through MDCT, a feature extractor that extracts a feature parameter of the first band signal from an MDCT coefficient output from the MDCT transformer; an extender that provides an extended MDCT coefficient for a second band signal based on the MDCT coefficient of the first band signal output from the MDCT transformer, a subband energy estimator that estimates subband energy of the second band signal with reference to information stored in the database based on the feature parameter.