In order to produce a music sketch (a combination of melody and accompaniment) in the beginning of musical composition, it is effective to use a composition means with high portability capable of inputting a melody in real time. In this paper, we propose a system that enables a beginner of musical composition or instrument easily and readily to create a music sketch by constructing a system capable of inputting a natural melody on a smartphone in real time in concert with accompaniment. In addition, we implemented the proposed system as an application to evaluate the proposed system through extensive use experiments by releasing it to the general public, showing the effectiveness of the proposed system in musical composition.