The present invention leaves the evidence of a contract on a blockchain with a simple method while having agreements made among the involved parties and maintaining the mode of one electronic signature per transaction. The present invention also leaves the evidence of a contract on a blockchain with a simple method while having agreements made among the involved parties, maintaining the mode of one electronic signature per transaction, and maintaining credibility. Transactions of an issuing-party terminal 1, an agreeing-party terminal 3a, and an agreeing-party terminal 3b involved in contract agreement are linked as a chain, and the agreeing-party terminal 3b at the end generates a transaction directed to the issuing-party terminal 1 who has issued the contract. Each of these terminals previously generates a key pair for encryption of the contract and transmits the encryption key to a terminal which is its transaction sender. Then, this sender terminal encrypts a contract to be included in a transaction of the user of the sender terminal using the encryption key of the receiver of the transaction.
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