Wireless local area networks (WLANs) have allowed us to deploy seamless and high capacity wireless systems easily and inexpensively. However, recently, channel interference between different services has become a serious problem because access points (APs) of WLANs are located too densely. In the carrier sense multiple access with collision avoidance used in WLANs, the hidden terminal (HT) problems occur depending on the distance between stations and the carrier sensing range. In the higher dense deployment mentioned above, the HT problems occur complicatedly. In this paper, we propose an AP cooperation system that detects the HT problems between stations (STAs). In our system, APs obtain the information of connected STAs from received frames and the HT problems are identified from the integration of the information obtained at different APs. The effectiveness is verified by simulations.