A signal processing apparatus includes: a separation processing unit that generates observed signals in the time frequency domain by performing the short-time Fourier transform on mixed signals as outputs, which are acquired from a plurality of sound sources by a plurality of sensors, and generates sound source separation results corresponding to the sound sources by a linear filtering process on the observed signals. The separation processing unit has a linear filtering process section that performs the linear filtering process on the observed signals so as to generate separated signals corresponding to the respective sound sources, an all-null spatial filtering section that applies an all-null spatial filter to generate signals filtered with the all-null spatial filter (spatially filtered signals) in which the acquired sounds in null directions are removed, and a frequency filtering section that performs a filtering process by inputting the separated signals and the spatially filtered signals.