Recently, interdisciplinary relation is emphasized on and various models are proposed. Since group theory is one of the important areas of modern mathematics, all the mathematics teachers for secondary school are familiar with it. Group theory, the theory of symmetries, are effectively applied to music or arts. In this paper, we understand the approximation model for gwaeyul of geomungo group theoretically to show the relation between mathematics and music(Korean music, in particular). This paper, in fact, proposes a group theoretic approximation model for gwaeyul of geomungo. The materials like this will be of help to teachers who try to integrate mathematics to other areas.