A stereo signal generating apparatus capable of obtaining stereo signals that exhibit a low bit rate and an excellent reproducibility. In this stereo signal generating apparatus (90), an FT part (901) converts a monaural signal (M′t) of time domain to a monaural signal (M′) of frequency domain. A power spectrum calculating part (902) determines a power spectrum (PM′). A scaling ratio calculating part (904a) determines a scaling ratio (SL) for a left channel, while a scaling ratio calculating part (904b) determines a scaling ratio (SR) for a right channel. A multiplying part (905a) multiplies the monaural signal (M′) of frequency domain by the scaling ratio (SL) to produce a left channel signal (L″) of a stereo signal, while a multiplying part (905b) multiplies the monaural signal (M′) of frequency domain by the scaling ratio (SR) to produce a right channel signal (R″) of the stereo signal.