We describe a method of adaptively controlling bandwidth allocation to flows for reducing the file transfer time of short flows without decreasing throughput of long-duration large flows. According to the rapid increase in Internet traffic volume, effective traffic engineering is increasingly required. Specifically, the traffic of long-duration large flows due to the use of peer-to-peer applications, for example, is a problem. Most conventional QoS controls allocate a fair-share bandwidth to each flow regardless of its duration. Thus, a long-duration large flow (such as a P2P flow) is allocated the same bandwidth as a short-duration flow (such as data from a Web page) in which the user is more sensitive to response time, i.e., file transfer time. As a result, long-duration large flows consume bandwidth over a long period and increase response times of short-duration flows, and conventional QoS methods do nothing to prevent this. In this paper, we therefore investigate a different approach, that is, a new form of bandwidth control that enables us to achieve better performance when handling short-duration flows while maintaining performance when handling long-duration flows. The basic idea is to tag packets of long-duration large flows according to traffic conditions and to give temporarily higher priority to nontagged packets during network congestion. We also show the effectiveness of our method through simulation.