WLAN signal based positioning is most popular in indoor localization, where GPS is hardly working inside buildings. In WLAN localization system, floor errors are occurred occasionally, which cause the serious side effects in indoor navigation system. In this paper, we introduce our floor-error correcting method. Simply, the correction method is based on Wi-Fi access point set similarity. We devised a new method with less floor errors than conventional system. We applied our proposed method to COEX, the most realistic case comparing with other researches. Therefore we expect that the result in this paper is meaningful to the researchers in same discipline.