Auxiliary digital data is sold and bought. A process for selling auxiliary digital data is performed with respect to a process for cancelling a limitation of use of the digital data. A process for buying the auxiliary digital data is performed. A process for receiving a selling request and a buying request is performed, and selling and buying transactions are concluded. The process of selling, the process of buying, and the process of selling and buying are each performed by transmitting and receiving data through a communication network.