This paper describes performance visualization with its purposes, significance, and our prototype system. Performance visualization provides us with a new user interface for music systems, a non-subjective explanation and judgment for musical expression, and a visual aid to understand, analyze, and compare performances and their musical structures. It will also introduce a more challenging research on designing a musical data mining interface not by tags nor by contents but by the mood. Other challenges include finding qualitative characteristics of music with analysis methods for information visualization and synesthesia between visualized and sonified information. We will describe which information should be shown on the visualized figures for expressive performance. Then we will show our prototype system to visualize expressive performance consisting of several instruments.