There is provided a signal processing apparatus for amplifying or attenuating, with respect to a signal in which a desired signal and another signal are mixed, the desired signal and the other signal at different ratios. The signal processing apparatus includes a separator that obtains an estimated first signal and an estimated second signal by receiving a mixed signal in which a first signal (for example, speech) and a second signal (for example, noise) are mixed and estimating the first signal and the second signal. Furthermore, the signal processing apparatus includes a gain adjuster that obtains a gain-adjusted mixed signal by receiving the estimated first signal and the estimated second signal.