In an encoder, a signal analysis unit performs signal analysis on an L channel signal and an R channel signal that constitute a stereo signal and generates a parameter used to determine a coding mode for each of an L channel and an R channel. A DMA stereo encoding unit encodes the L channel signal and the R channel signal by using a coding mode common to the L channel signal and the R channel signal. At this time, the DMA stereo encoding unit determines the common coding mode by selecting, out of the L channel and the R channel, the one that has a lower ratio of energy of an environmental sound component to the entire energy of the channel and using the parameter of the selected channel.