We have been teaching computer music to hearing impaired students of Tsukuba College of Technology for six years. Although students have hearing difficulties, almost all of them show an interest in music. Thus, this has been a challenging class to turn their weakness into enjoyment. We thought that performance visualization is a good method for them to keep their interest in music and try cooperative performances with others. In this paper, we describe our computer music class and the result of our preliminary experiment on the effectiveness of visual assistance. Though it was not a complete experiment with a sufficient number of subjects, the result showed that the show-ahead and selected-note-only types of performance visualization were necessary according to the purpose of the visual aid.