1. A linkage support system that links related messages among a plurality of messages sent and received among a plurality of communication terminals comprising; an information acquisition unit that acquires message types of said plurality of messages, character strings included in said plurality of messages, and amount of data flow when said character strings are transferred; an unnecessary character strings exclusion unit that excludes character strings that are not required as keywords for linkage processing, based on the information acquired by said information acquisition unit; and a processing order determination unit that determines an order for using a plurality of character strings from which unnecessary character strings have been excluded by said unnecessary character strings exclusion unit.