RFID technology is one of automatic identification technologies. In current RFID systems, RFID data are managed and processed by a middleware. In the near future, when RFID technology will be applied to large scale warehouses, airports, or seaports, it is necessary that wireless sensors integrated a RFID reader construct wireless sensor network because of difficulties of building wired network infrastructure. However adjacent RFID readers in wireless sensor network can generate many duplicate data at the same time. These duplicate data transmissions may waste the energy of sensor nodes integrated a RFID reader and cause network congestions. So we propose in-network phased filtering mechanism for a large-scale RFID inventory application. In this paper, we describe our idea to reduce a processing time of filtering and confirm the idea by a simulation.