Research on Internet of Things (IoT) application systems, which store data collected by various sensor devices to a cloud and utilize the data through statistical analysis and machine learning, has been conducted extensively. Several platforms have been proposed to support the development of IoT application systems, which enable system administrators to easily manage edge hardware and group sensors. However, conventional development support platforms do not provide functions for secure data management, such as encryption processing when handling confidential information and flexible access control among members of a research group. In this paper, we propose a mechanism for securely and easily encrypting data and sharing data using a Secure Configuration Server. We have been developing "SINETStream," an IoT application development support tool provided by National Institute of Informatics, Japan. The secure configuration server can co-work with the SINETStream-based IoT systems. This mechanism enables easy sharing of confidential information by securely managing data encryption keys within the system, which has been complicated with conventional methods. In addition, our mechanism provides users with confidential information in an encrypted form, so that only authorized users can view the data.