An audio signal decoding device which is operable to decode a downmix channel signal obtained by downmixing audio channel signals, into the audio channel signals, said audio signal decoding device comprising: a QMF filterbank operable to transform the downmix channel signal into a time-frequency representation over plural frequency bands segmented along a frequency axis and operable to transform the audio channel signals into time-frequency representations, each of the audio channel signals being quantized to difference signals (LIf(t), Rlf(t)); a low-pass filter unit operable to limit the transformed downmix channel signal to a bit-rate dependent crossover frequency; a left mixer and a right mixer operable to premix the transformed downmix channel signal limited to the bit-rate dependent crossover frequency and the transformed audio channel signals so as to generate premix channel signals; a transient detecting unit operable to detect a transient characteristic of the downmix signal; a high-pass filter unit operable to limit the downmix signal only to a band higher than the bit-rate dependent crossover frequency; a reverberation generating unit operable to generate a reverberation component based on a degree of transient characteristic of the detected downmix signal and a degree of correlation between the audio channel signals indicated by a spatial audio signal, and to add the reverberation component to the downmix signal limited only to the high band; a phase adjusting unit (306, 308) operable to adjust a phase of the downmix signal with the reverberation component added, based on information on a phase difference between the audio channel signals; a mixing unit (310, 312) operable to mix the downmix channel signal after the phase adjustment, with the generated premix channel signals so as to generate mixed channel signals; and an energy adjustment unit (314, 316) operable to adjust energy of the mixed channel signals for each of the frequency bands segmented along the frequency axis; and a mixed channel signal transformation unit operable to transform the mixed channel signals with the adjusted energy into the audio channel signals which are time signals, wherein the total number of audio channel signals equals two. a QMF filterbank operable to transform the downmix channel signal into a time-frequency representation over plural frequency bands segmented along a frequency axis and operable to transform the audio channel signals into time-frequency representations, each of the audio channel signals being quantized to difference signals (LIf(t), Rlf(t)); a low-pass filter unit operable to limit the transformed downmix channel signal to a bit-rate dependent crossover frequency; a left mixer and a right mixer operable to premix the transformed downmix channel signal limited to the bit-rate dependent crossover frequency and the transformed audio channel signals so as to generate premix channel signals; a transient detecting unit operable to detect a transient characteristic of the downmix signal; a high-pass filter unit operable to limit the downmix signal only to a band higher than the bit-rate dependent crossover frequency; a reverberation generating unit operable to generate a reverberation component based on a degree of transient characteristic of the detected downmix signal and a degree of correlation between the audio channel signals indicated by a spatial audio signal, and to add the reverberation component to the downmix signal limited only to the high band; a phase adjusting unit (306, 308) operable to adjust a phase of the downmix signal with the reverberation component added, based on information on a phase difference between the audio channel signals; a mixing unit (310, 312) operable to mix the downmix channel signal after the phase adjustment, with the generated premix channel signals so as to generate mixed channel signals; and an energy adjustment unit (314, 316) operable to adjust energy of the mixed channel signals for each of the frequency bands segmented along the frequency axis; and a mixed channel signal transformation unit operable to transform the mixed channel signals with the adjusted energy into the audio channel signals which are time signals, wherein the total number of audio channel signals equals two.