A data sales mediation system having a user terminal, a data sales mediation server and a data sales server, said user terminal comprising: means for sending to said data sales mediation server a registration request signal including user identification information and an online access code unique to data stored in a storage medium; means for encrypting and storing a key code included in a registration confirmation signal received from said data sales mediation server; and means for using data by using the key code, said data sales server comprising: first means for storing information on the key code corresponding to said online access code one to one; and means for sending information on the key code to said data sales mediation server in predetermined timing, and said data sales mediation server comprising: storage means for user information; second means for storing information on the key code corresponding to said online access code one to one; means for determining the number of the user terminals for storing the key code corresponding to said online access code one to one respectively; means for acquiring the information on the key code from said data sales server and storing it in said second means for storing; means for determining whether or not user information corresponding to said user identification information is stored in said storage means; and means for, in the case where the registration request signal has a registration confirmation signal sending requirement, acquiring the key code corresponding to the registration request signal one to one from said second means for storing and sending to the user terminal said registration confirmation signal including the key code, wherein said registration confirmation signal sending requirement includes a condition that user information corresponding to said user identification information is stored in said storage means, a condition that the information on the key code corresponding to said online access code one to one is stored in either said first or second means for storing, and a condition that the number of the user terminals for storing the key code corresponding to said online access code one to one is smaller than a predetermined number. means for sending to said data sales mediation server a registration request signal including user identification information and an online access code unique to data stored in a storage medium; means for encrypting and storing a key code included in a registration confirmation signal received from said data sales mediation server; and means for using data by using the key code, first means for storing information on the key code corresponding to said online access code one to one; and means for sending information on the key code to said data sales mediation server in predetermined timing, and said data sales mediation server comprising: storage means for user information; storage means for user information; second means for storing information on the key code corresponding to said online access code one to one; means for determining the number of the user terminals for storing the key code corresponding to said online access code one to one respectively; means for acquiring the information on the key code from said data sales server and storing it in said second means for storing; means for determining whether or not user information corresponding to said user identification information is stored in said storage means; and means for, in the case where the registration request signal has a registration confirmation signal sending requirement, acquiring the key code corresponding to the registration request signal one to one from said second means for storing and sending to the user terminal said registration confirmation signal including the key code, wherein said registration confirmation signal sending requirement includes a condition that user information corresponding to said user identification information is stored in said storage means, a condition that the information on the key code corresponding to said online access code one to one is stored in either said first or second means for storing, and a condition that the number of the user terminals for storing the key code corresponding to said online access code one to one is smaller than a predetermined number.