A parallel computer operates to reduce data held by a plurality of nodes. Each node constituting a parallel computer transfers the data divided into n to other nodes, and each node summarizes the respective 1/n data and operates, then a plurality of nodes transfer the respective operation result to a summarizing node. Since all the nodes execute operation for divided data respectively, reduction processing time can be decreased. And more nodes (network adapters) can participate in data transfer through the first and second data transfers, high-speed transfer processing can therefore be implemented, and transfer time can be decreased.