A signal processing method includes separating a mixed sound signal in which a plurality of excitations are mixed into the respective excitations, and performing speech detection on the plurality of separated excitation signals, judging whether or not the plurality of excitation signals are speech and generating speech section information indicating speech/non-speech information for each excitation signal. The signal processing signal also includes at least one of calculating and analyzing an utterance overlap duration using the speech section information for combinations of the plurality of excitation signals and of calculating and analyzing a silence duration. The signal processing signal further includes calculating a degree of establishment of a conversation indicating the degree of establishment of a conversation based on the extracted utterance overlap duration or the silence duration.