1. A linkage support system that links related messages among a plurality of messages sent and received among a plurality of communication terminals, said linkage support system 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; and a processing order determination unit that determines a processing order of the character strings to be linked for using a plurality of character strings from which unnecessary character strings have been excluded by said unnecessary character strings exclusion unit; and a central processing unit that operates the information acquisition unit, unnecessary character strings exclusion unit, and processing order determination unit, wherein the processing order determination unit determines the linkage processing order of the character strings to be linked so that character strings with smaller total amount of data flow is used first, based on the amount of data flow acquired by the information acquisition unit.