QoS (Quality of Service) control in WLAN (IEEE802.11 Wireless LAN) is becoming increasingly important because WLAN is widely deployed as an access network and also plays a key role in providing seamless QoS communication between wired networks and wireless terminals. Although previous research has attempted to increase total throughput or available bandwidth in WLAN, few studies have treated individual TCP/UDP flow QoS. EDCA in IEEE802.11e might provide prioritized QoS functions that would partially address this problem. However, in uplink flow, which is defined as data moving from a terminal toward an Access Point, EDCA has limitations. These manifest themselves both across classes and in differentiated QoS control between terminals in the same class. Furthermore, 802.11e requires modification of terminals as well as other alterations proposed by other researchers. Instead of 802.11e or other modifications of 802.11, we propose an approach to controlling QoS that requires no terminal modifications or installation of additional software/hardware. The proposed idea is MAC-frame Receiving-Opportunity Control (ROC), in which a MAC (Media Access Control) frame is completely delivered only if it has sufficiently high priority; otherwise either the MAC frame is discarded or an Acknowledge (ACK) to the frame is not sent. The frame that was discarded is forced to accept a back-off waiting time for retransmission, consistent with 802.11DCF. This results in QoS degradation for low priority flows and eventually results in QoS improvements for high priority flows. Performance evaluation shows that the ROC causes some performance degradation in total WLAN throughput but can achieve not only QoS priority control but also arbitrary throughput performance. In particular, the ROC (in the MAC layer) can also permit different throughputs for high priority and low priority flows, conditioned on control processes in other layers. These may include rate adaptation (in the MAC layer) and TCP congestion control (in the TCP layer).