PROBLEM TO BE SOLVED : To provide a transaction mediation device, a transaction mediation method, and a transaction mediation program that realize a client' s request to the utmost in a real estate deal, and expand the variety of services of a real estate agency to enhance superiority.SOLUTION : A real estate brokerage system 1 includes : a mediation server having a transaction condition setting section for setting transaction conditions that are transaction objects; and a guarantee system having a transaction execution section for executing a transaction on the basis of an own guarantee. The transaction condition setting section randomly determines a part of the transaction conditions containing a brokerage fee or a discount rate associated with the transaction objects, and sets the transaction conditions. Information related to the transaction conditions contains information on any of a function, a random number list or an algorithm used for random calculation. The guarantee system issues a secret key to be utilized for an electronic signature of a transaction mediated through a public address, which is generated from a public key in public key cryptography and capable of specifying a public key when paired with the public address and the public key for identifying specified users.SELECTED DRAWING : Figure 2COPYRIGHT : (C)2019, JPO&INPIT