A sound signal processing apparatus includes an observed signal analysis unit that receives as an observed signal a sound signal for channels obtained by a sound signal input unit formed of microphones and estimates a sound direction and a sound segment of a target sound which is sound to be extracted and a sound source extraction unit that receives the sound direction and sound segment of the target sound estimated by the observed signal analysis unit and extracts the sound signal for the target sound. The observed signal analysis unit includes a short time Fourier transform unit that generates an observed signal in time-frequency domain by applying short time Fourier transform to the sound signal for the channels received and a direction/segment estimation unit that receives the observed signal generated by the short time Fourier transform unit and detects the sound direction and sound segment of the target sound.