This paper describes a method for change detection of conditions of indoor environments using the analysis of strength of wireless LAN signals. The method uses the characteristics that the signal strength of the wireless LAN access points and clients. This paper focuses on the implementation of measurement system of the wireless LAN signals using micro controllers for the portability and scalability, and the online analysis of the change detection of the wireless LAN signals for the surveillance applications. We construct a method for online analysis using multiple PCs and RT middleware framework. We show the feasibility of the methods through the system integration and experimental results.