A method and apparatus for encoding audio data and a method and apparatus for decoding audio data, which can generate and decode, respectively, scalable lossless streams and which can shorten the time necessary to generate and decode lossless streams. A lossy-core encoder unit performs lossy compression on an input audio signal, generating a core stream. A simplified lossy-core decoding unit decodes only spectral signals of a specified band, e.g., a lower frequency band to generate a lossy decoded audio signal. A subtracter subtracts a lossy decoded audio signal from the input audio signal delayed to generate a residual signal. A rounding-off unit performs a process of rounding off the number of bits constituting the residual signal by eliminating the residual sign bit without loss of precision. A lossless-enhance encoder unit performs lossless compression on the residual signal to generate an enhanced stream. A stream-combining unit combines the core stream and the enhanced stream to generate a scalable lossless stream.