1. A sorting system comprising: a plurality of input nodes; one output node; and a shared external storage unit connected between each of said input units and said output unit, wherein: each of said input units comprises: a first buffer; means for storing sorting target data in said first buffer; means for internally sorting data in said first buffer in accordance with a predetermined sorting rule; and means for storing as a sorted string an internally sorted result in said shared external storage unit; and said output node comprises: a second buffer; means for storing in said second buffer the sorted string stored in said shared external storage unit; means for merging a plurality of sorted strings stored in said second buffer, in accordance with the predetermined sorting rule; and means for outputting a merged result as a sorted result.