Sensor networks have been considered as a base technology for constructing ubiquitous computing environment. However, most researches about sensor networks have not focused on ubiquitous computing but mainly focused on efficiency of sensor networks itself. Ubiquitous computing requires sensor networks to be accessed in a transparent and abstract manner from the outer world. However the requirements cannot be easily achieved because of physical restrictions of sensor networks and centralized data processing scheme. In this paper, we have designed a sensor overlay network that enables ubiquitous computing applications to consider sensor networks as an ordinary component of ubiquitous computing environment. Virtual sensors and actuators on the overlay network cooperate with each other and provide intelligent decision that was impossible to be implemented with current physical sensor networks. The proposed overlay network also provides dynamic service composition and device integration essential to ubiquitous computing. To evaluate the usefulness of the designed system, the overlay network is implemented and simulated on J-Sim simulator.