For effectively rendering surface details, many texture-based methods such as normal mapping and displacement mapping were used to model detailed structures on a surface. Although self-shadow can be achieved by those techniques, most of them support hard shadow or ad-hoc approximation of soft-shadow for the self-shadow effects. In this paper, we present a simple method to render soft shadow casted by area lights on displacement mapped surfaces in real-time rendering applications such as 3D game engines. Our method first computes a visibility cone map, which stores the average visible ratio of each texel toward a certain direction. Then, during the rendering, the intensity of each pixel on the texture mapped surfaces can be calculated from the overlap between the projected lighting area and the projected visible cone.