In this paper, we propose a WiFi-based stop notification system. The users of the system inform their destination stops to the system, then the system provides subway navigation services to the users in real-time. To construct the system, we have collected WiFi fingerprints at 15 lines for 536 Seoul subway stops. Also, we have implemented a WiFi-based subway stop notification system for Seoul subways in Korea. We have tested the accuracy of our system, Nerimi, at Seoul subways. Among the 536 stops, the system could detect the subway stops with over 90% accuracy. Thus, if we can capture the WiFi signals at every stop of the subways, using the WiFi signals will be highly eligible for building a subway navigation system.