An audio signal in which a first audio component and a second audio component are mixed is inputted. The audio signal is transformed into a time-frequency signal representing relation between time and signal frequency. The time-frequency signal is divided into a plurality of bands. A time-frequency signal in a band, in which influence from the second audio component is small among the plurality of bands, is factorized into activity vectors composing a first activity matrix. The time-frequency signal transformed from the audio signal is factorized into activity vectors composing a second activity matrix using the first activity matrix as a teacher activity.