A method for processing an audio signal, comprising the steps of extracting an ancillary signal for generating the audio signal, an extension signal included in the ancillary signal, and header identification information indicating whether a header is included in the ancillary signal from a received bit stream, reading length information of the extension signal from the header if the header is included in the ancillary signal according to the header identification information, skipping decoding of the extension signal or not using a result of the decoding based on the length information, and generating the audio signal using the ancillary signal. Accordingly, in case of processing the audio signal by the present invention, it is able to reduce a corresponding load of operation to enable efficient processing and enhance a sound quality.