We present an interactive method of visualizing both manifold and non-manifold implicit surfaces. The implicit surfaces are directly visualized at interactive frame rates independent of surface complexity by the hardware-accelerated volume rendering method. Although conventional implicit surfaces can represent only two-manifold surfaces, we have developed a technique of representing nonmanifold surfaces as implicit surfaces by applying segmented distance field. Implicit surfaces represented by this definition can be rendered by preintegrated volume rendering using vertex generation diagrams as pre-integration table. We also developed a system for visualizing the implicit surfaces and have confirmed that it can render surfaces at sufficient quality and speed.