This paper reports a newly designed visualization API (Application programming Interface) for a Cartesian and octree data structure which allows us. to render and visualize the data efficiently. It is designed that this visualization API is consists of a window frame class, a scene-graph class and a visualization function class. The window frame class is constructed from wxWidgets, which is open source GUI system, so as to be operated on major platforms such as Linux, Windows and MacOS X. As concerns the scene-graph class, we developed an original API instead of the standard sene-graph library like VTK because of seeking of the rendering efficiency and supporting the special data structure. Based on this API, a rendering application was developed and evaluated. This data viewer will be delivered from RIKEN.