Context-aware computing offers attractive services by collecting information from various sensors, and the services will be adapted according to the current condition of the real world. However, the danger of overflowing personal information is also getting high because the data retrieved by the sensors usually contain privacy information. In this paper, we study on the tradeoff between privacy and the quality of services, and we present the design of our system that we are currently developing. Our proposed system controls the tradeoff according to a user's requirement, and guarantees the trustworthy of the management of privacy information.