Licensed band communication systems have recently begun trying to utilize unlicensed bands. Due to lack of coordination, however, this spectrum utilization results in strong interference to other unlicensed band equipment (UBE). This problem highlights the need for coexistence scenario that will prevent UBE performance degradation. In this paper, we propose a new access control mechanism for downlink cellular networks using the unlicensed band. First, we analyze the impact of the cellular networks on the UBE outage performance. Based on the analysis, we then obtain a decision threshold for the unlicensed band access control. In the proposed control mechanism, the decision threshold is adaptively controlled by considering the density of the UBE. The numerical results demonstrate that the proposed access control mechanism guarantees the UBE's target performance under various network circumstances.