Abstract: A sound signal processing device that obtains a two-channel stereo encoding target signal to be subjected to stereo encoding from a two-channel stereo input sound signal, the sound signal processing device including a signal mixing unit that obtains, as an encoding target signal, for each channel, a signal obtained by performing weighted addition on an input sound signal of the channel and an input sound signal of the other channel, the signal being closer to the input sound signal of the channel as the two-channel stereo input sound signal is likely to be a single sound source.