An apparatus for processing a signal and method thereof are disclosed. The present invention includes receiving a downmix signal generated from a plural-channel signal, mix information and phase shift information on the plural-channel signal, upmixing the downmix signal into the plural-channel signal by applying the mix information to the downmix signal, and generating an original plural-channel signal by shifting a phase of at least one channel of the plural-channel signal based on the phase shift information. According to the present invention compensate a reconstructed original plural-channel signal using compensation information (phase shift information), thereby compensating a phase or a gain lost in the plural-channel signal reconstructed by upmixing using mix information.