An effort to deploy security devices by the network provider has been increasing as the network is being exposed to various types of network attacks. However, network providers are incapable of handling all types of attacks as each security device is designed for a certain purpose. If an attack breaks out, only one particular device becomes busy in terms of resource usage while others being idle. Moreover, it is hard to adjust a level of security service with respect to the importance of network flow. To address these issues, we propose a new security solution, QoSE, which provides adaptive security services based on Network Function Virtualization (NFV). QoSE provides a capability to manage resource usage that the network flow is not concentrated on a specific node. We design QoSE considering a distributed NFV environment to avoid a single point of failure and a bottleneck problem. Our proposed solution has also shown a quick recovery from fault situation. In addition, we provide a novel resource optimization algorithm to operate security services efficiently. We have implemented a prototype system to verify our ideas and have checked that QoSE shows reasonable performance compared with a common device.