Everyone needs to be relaxed in some way. Previous studies show that listening to the combination of musical pieces and natural sounds effectively provide listeners with a relaxed feeling. In this study, we advocate an Interactive Genetic Algorithm (IGA) that composes musical piece suited to natural sound. IGA is a method to create media content that fits users’ feelings based on their evaluation. Considering the fatigue and burden brought by subjective evaluation of users, the ratio of alpha-wave power in Electroencephalography (EEG) is used as fitness in this study. Based on the proposed IGA, we built a system taking musical notes, entire volume, and tempo as variables to adjust. With the IGA system, a listening experiment was carried out to investigate the fundamental efficiency of the proposed IGA. As a result, a marginal increase in the mean fitness was observed between the initial and final generations. However, no significant difference was observed in the subjective evaluation step after the search step.