To provide a device and program for association type transaction mediation and their method which mediate transactions when ≥3 users have transactions, etc., in series a of secondhand articles.

An information processing means of a computer is used to input through an input means of the computer selling information on an article and buying information on an article from a plurality of users including at least one user who desires to sell and buy the article at the same time and determines whether a combination of the selling and buying of the article is realized in scsies among ≥3 persons including at least the user who desires to sell and buy the article at the same time according to the inputted information. When it is determined that the combination of the selling and buying of the article is actualized in series among the ≥3 persons, the users are notified through an output means of the computer that the selling and/or buying of the article is actualized.