Vehicle-to-Everything (V2X) communication enhances the capability of autonomous drivingthrough better safety, efficiency, and comfort. In particular, sensor data sharing, known as cooperativeperception, is a crucial technique to accommodate vulnerable road users in a cooperative intelligenttransport system (ITS). In this paper, we describe a roadside perception unit (RSPU) that combinessensors and roadside units (RSUs) for infrastructure-based cooperative perception. We propose asoftware called AutoC2X that we designed to realize cooperative perception for RSPUs and vehicles.We also propose the concept of networked RSPUs, which is the inter-connection of RSPUs alonga road over a wired network, and helps realize broader cooperative perception. We evaluated theRSPU system and the networked RSPUs through a field test, numerical analysis, and simulationexperiments. Field evaluation showed that, even in the worst case, our RSPU system can delivermessages to an autonomous vehicle within 100 ms. The simulation result shows that the proposedpriority algorithm achieves a wide perception range with a high delivery ratio and low latency,especially under heavy road traffic conditions.