The purpose of this study is to offer a holistic description of the role demanded of costume designers to provide qualitative improvement of costume designs for small- and medium-sized musicals. The study accomplishes this by analyzing the relationship between the play and costume design in musicals and also by examining the process and environments in which costumes of small-sized theater companies` original musicals are produced. The methods of study are empirical research of literature related to dress, stage art, and culture & arts from both domestic and foreign publications; research of visual materials related to plays; and production and analysis of theatrical performances in which the researcher has participated as a costume designer. The results of the study are as follows. First, professionalism of the play and visual perfection of the work may be enhanced by costume designers actively participating in meetings with staff from each field of the play. Second, by taking advantage of the unique aspects of the work environment of small-sized theater companies` small- and medium-sized original musicals, such as harsh situations of regular and periodical meetings with all staff member, the designer may consider the director`s intentions and concepts of the play, but should display more creative and autonomous design abilities. Third, because the costume designers of small- and medium-sized original musical plays are given a small budget and short production periods, the ability to systematically manage budget and production periods and the ability to flexibly handle unexpected incidents during the play is essential.