This paper proposes an Augmented Reality (AR) system to assist operation and maintenance tasks in an indoor environment, such as a nuclear fusion reactor and its building. AR is a technology that enhances real information by adding 3D virtual objects, images, sounds, or movies via a web camera. The AR system often uses "markers" such as QR code to detect the place where the virtual content should appear. However, these markers are unnatural and they can disturb the scenery. We propose an AR system that can detect natural markers, which provides AR content via a network. This system stores the information related to markers and virtual objects on a server. A device connected to this system automatically downloads this content so that the user can watch the AR content via a web camera. We add a real-time numerical simulation function that allows us to simulate physical phenomena by touching AR contents. It also enables us to observe simulation results by downloading a movie of numerical simulation results from the simulation server. Overall, this system allows us to watch the same content with multiple devices and to simulate physical phenomena using various parameters.