While “conducting” is briefly mentioned as the class methodology in the secondary school textbook instruction and the senior school curriculum guidelines for schools for the intellectually disabled, nothing has been studied on how a conductor would be affecting playing music and others. Therefore, this article focuses on the necessity of a conductor at a senior high school for the intellectually disabled, reporting on the author's music class having many students who can sing in chorus and play in concert. Because of public education, the class was practiced according to the above literary works of Ministry of Educations. As a result of comparing lessons with and without a conductor each other, it was observed that the conductor contributed to students' accurate ensemble and the music expression which was active act. This was not established without involving a man (a conductor) except students themselves. In conclusion, the conductor was not only required for ensemble but also contributing to acquisition of sociality for the intellectually disabled to learn.