Sensor devices of many types have come into widespread use, producing huge amounts of data. If the observed data can be aggregated via the internet, then new applications can be produced to provide useful information for wide areas. As described in this paper, we propose a time-slot-based data aggregation mechanism on a structured p2p network. The proposed mechanism uses routing tables of the structured p2p network to achieve scalable data collection. In addition, the proposed system uses a time-slot idea to aggregate the observed data accurately. Nodes in the system record the observed data as time slot values. Then the nodes send the time slot values to other nodes. When nodes receive the time slot values, the nodes aggregate the received values as data aggregation results. As described in this paper, we explain an algorithm and a communication protocol for the proposed data aggregation mechanism. Furthermore, using experimentally obtained results, we demonstrate that the proposed mechanism achieves more accurate data aggregation than existing mechanisms.