The purpose of this study is to analyze library space use patterns through users’ smartphone WiFi. This study is applied a method to detect WiFi signal of users’ smartphone to analyze the in-library wayfinding of users. The library usage data were collected for four months in a library in Seoul, Korea. The results show that the average 37.9% of library users revisits the library the next month. Half of users stay under 7 minutes in the library. Users mainly visit the library between 2 and 3 o’clock, and few users visit the library after 5 pm on weekends. The floor moving pattern result shows that the co-visit rate between the third and fourth floor is higher than others, in that these two floors are mainly composed of book shelves. These results indicate that the method to detect the WiFi signal for spatial pattern analysis could be more effective than observation which was used in previous research. It, therefore, is expected that this method would be applied in other libraries to analyze and enhance the library space usage.