Recently, unmanned aircraft systems (UASs) have attracted attention as a new avenue for commercial services. Using the flexible mobility of unmanned aircrafts (UAs), commercial services can be operated in wide areas. However, there is a problem in the wireless communication between the UA and its ground station. When several UASs are operated within the neighboring airspace, wireless-communication conflicts occur. One of the most effective solutions for this issue is to decide the communication schedule using a time-division multiple access (TDMA) scheme. Furthermore, by spatially reusing the time slot, numerous UAs can be operated within the neighboring airspace, in a limited frequency band. In this paper, we propose an efficient time-slot allocation for enhancing the frequency resource utilization. Our proposed scheme determines the time-slot allocation considering the time-slot spatial reuse, using a virtual cell based space partitioning method. In addition, we consider the influence of the UA mobility on the network to decide the parameters for the proposed resource allocation system. The effectiveness of our proposed resource allocation is evaluated through computer-based simulation.