Recently, many kinds of real-time applications have become available over IP networks. It is important to measure network performance for such applications before making use of real applications. The authors developed a general purpose traffic measurement tool for IP networks. This system can generate any kind of traffic flexibly and calculate network performance such as throughput, delay and loss rate according to the packers to be observed. In this paper, the concept and implementation of this tool are described in detail. As an example of network measurements, we also conducted subjective assessment tests for Internet telephony and demonstrated the validity of this tool.