A method of processing a signal is disclosed. The present invention includes receiving (a) a downmix signal being generated from plural-channel signal and (b) spatial information indicating attribute of the plural-channel signal, in order to upmix the downmix signal; obtaining inter-channel phase difference(IPD) coding flag indicating whether IPD value is used to the spatial information from a header of the spatial information; obtaining IPD mode flag indicating whether the IPD is used to frame of the spatial information from the frame based on the IPD coding flag; obtaining the IPD value from a parameter band in the frame based on the IPD mode flag; generating plural-channel signal by applying the IPD value to the downmix signal, wherein the spatial information is divided by header and a plurality of the frame and wherein the IPD value indicates phase difference between two channels of the plural-channel signal and wherein the parameter band is at least one sub-band of frequency domain including the IPD value.