This paper describes a method for the detection of the state of the indoor environment using the change of the strength of the wireless LAN signals. Data acquisition of the physical location of the target person by a monitoring system is important for the person's safety and reliability. For the monitoring system, camera devices are widely used because of the width of view; however, it is difficult to apply the camera system to monitor the person in the environment because of the privacy concerns. The sensing system that can obtain the wide range sensing area but cannot obtain the shape of the target objects or persons directly is desired. We focus on change of the pattern of the wireless LAN signal strength based on the singular spectrum transformation of the time series of of the wireless LAN signal strength. We apply the singular spectrum transformation to detect the change of the strength of wireless LAN signal patterns. Experimental results show the feasibility of the method.