With the advancement of communication technologies, multiple wireless local area networks (WLANs) have been often deployed in the same building for the Internet access services. Usually, their wireless access-points (APs) have been independently managed and operated by different groups such as departments or laboratories in a university or a company. Then, a WLAN user host can detect signals from multiple APs, which may increase the interference and the operation cost, and degrade the communication performance. As a result, the number of active APs should be minimized by aggregating the physical APs allocated in the field into limited ones by introducing virtual APs. In this paper, we define this AP aggregation problem for WLANs that aggregates the APs into the minimum number of active APs, and prove the NP-completeness of its decision problem. Then, we propose the heuristic algorithm that is composed of five phases. The effectiveness of our proposal has been verified through simulations in two instances.