There is an emerging requirement for real-time flow-based traffic monitoring, which is vital for quality of service (QoS) monitoring, anomaly monitoring, and traffic engineering in Internet Service Provider networks. In this report, we discuss a distributed architecture for real-time and scalable flow monitoring tools and implement a prototype tool using Network Processor (NP). The contributions of the present paper are two-folds. First, we clarify the elemental functions that are required and commonly used by passive flow monitoring tools, and refine a distribution-processing architecture for combining these functions based on our previous work in order to cope with heavy-load input traffic. Second, we implement the packet distribution component, which plays a core role and might be bottleneck in the proposed architecture, by using pipeline-mode NP hardware, and experimentally prove its applicability to 10 Gbit/s-class input traffic.