We propose a method for the sound source separation using a stereo music signal produced by three instruments while a listener moves. We donot use the position information of players and a listener because it is unknown in a general stereo music signal. We made a histogram using transitions of the ratio between left and right volume, and estimated the position where two sound sources are at the same place using a clustering method. We track the sound source as the position between sound sources as a listener moves to seperate the sound source. We show the e®ectiveness of the method by using Vivaldis Four Seasons recorded in an active environment.