PROBLEM TO BE SOLVED : a customer of real estate sales management system capable of suitably supporting the, property management method, and a program. SOLUTION : the customer purchases and sales contract with provisions for agreement, sales contract with accompanying the special provisions, the real estate for a building and contract periodically creditworthiness leasehold registered information relating to the initial information registration part 120, with the special provisions in the sales contract, determined by the building contractor leasehold leasehold periodically during the period that the irrevocability is determined it is possible, and the initial information registration part 120, whether the building lease contract irrevocability periodically, with special provisions and sales contract irrevocability irrevocability information regarding whether the information irrevocability 150 and, during the period leasehold, when irrevocability information is not registered and issues a notification section 180, is provided. Selected drawing : fig. 2