This paper proposes a selective sound segregation model for separating target musical instrument sound from the mixed sound of various musical instruments. The model consists of two blocks: a model of segregating two acoustic sources based on auditory scene analysis as bottom-up processing, and a selective processing based on knowledge sources as top-down processing. Two simulations were carried out to evaluate the proposed model. Results showed that the model could selectively segregate not only the target instrument sound, but also the target performance sound, from the mixed sound of various instruments. This model, therefore, can also be adapted to computationally model the mechanisms of a human's selective hearing system.