A method for extending bandwidth of an audio signal, the method comprising: performing (S510; S600) a modified discrete cosine transform, MDCT, process on an input signal to generate a first transform signal; generating (S520, S530; S610, S620) a second transform signal and a third transform signal on the basis of the first transform signal; generating (S540; S630, S640, S650) normalized components and energy components of the first transform signal, the second transform signal, and the third transform signal therefrom; generating (S560; S670) an extended normalized component on the basis of the normalized components of the first transform signal and the second transform signal and generating (S550; S660) an extended energy component from the energy components; generating (S570; S680) an extended transform signal on the basis of the extended normalized component and the extended energy component; and performing (S580; S690) an inverse MDCT, IMDCT, process on the extended transform signal, wherein the second transform signal is a signal obtained by double spectral stretching (S530; S620) of the first transform signal to an upper frequency band, wherein the third transform signal is a signal obtained by reflecting (S530; S620) the first transform signal with respect to a first reference frequency which is an uppermost frequency of the first transform signal, and wherein the extended energy component is the energy component of the first transform signal in a first energy section with a frequency bandwidth of K in which the first transform signal is defined, is synthesized by weighting the energy component of the second transform signal and the energy component of the third transform signal in a second energy section which is an upper section with a bandwidth of K/2 from the uppermost frequency band of the first energy section, and is the energy component of the second transform signal in a third energy section which is an upper section with a bandwidth of K/2 from an uppermost frequency band of the second energy section. performing (S510; S600) a modified discrete cosine transform, MDCT, process on an input signal to generate a first transform signal; generating (S520, S530; S610, S620) a second transform signal and a third transform signal on the basis of the first transform signal; generating (S540; S630, S640, S650) normalized components and energy components of the first transform signal, the second transform signal, and the third transform signal therefrom; generating (S560; S670) an extended normalized component on the basis of the normalized components of the first transform signal and the second transform signal and generating (S550; S660) an extended energy component from the energy components; generating (S570; S680) an extended transform signal on the basis of the extended normalized component and the extended energy component; and performing (S580; S690) an inverse MDCT, IMDCT, process on the extended transform signal, wherein the second transform signal is a signal obtained by double spectral stretching (S530; S620) of the first transform signal to an upper frequency band, wherein the third transform signal is a signal obtained by reflecting (S530; S620) the first transform signal with respect to a first reference frequency which is an uppermost frequency of the first transform signal, and wherein the extended energy component is the energy component of the first transform signal in a first energy section with a frequency bandwidth of K in which the first transform signal is defined, is synthesized by weighting the energy component of the second transform signal and the energy component of the third transform signal in a second energy section which is an upper section with a bandwidth of K/2 from the uppermost frequency band of the first energy section, and is the energy component of the second transform signal in a third energy section which is an upper section with a bandwidth of K/2 from an uppermost frequency band of the second energy section.