Students often say that "I don't understand the meaning." For those students, figures and diagrams are powerful aid to understand the meaning, and animation has still great potential since capable students and math teachers are usually good at operating the image in their mind. Using animation, teacher s can realize their own vivid image on the screen. If students succeed to intern alize the image of concepts, they will obtain the ability to operate in their mind, and th ey will be able to predict the rough result. In this paper, I will discuss the role of interactive animation with several examples at our web site.