Natural disasters frequently occur around the world and can cause serious damage to our daily lives. To minimize such damage, it is essential to assess the conditions of affected areas promptly. This paper proposes a novel system that automatically issues safety alerts regarding building damage. The proposed system utilizes preinstalled Wi-Fi access points and smartphones, which are commonly used in usual life. It collects spatial data under normal conditions using Wi-Fi RTT and compares it with data collected immediately after a disaster. By analyzing these differences, the system detects anomalies through a method we developed, called Floor Map Investigation, which is based on distance relationships. We evaluate the effectiveness of the proposed system through computer simulations. The results demonstrate that our system can rapidly and accurately detect anomalies caused by building damage.