The present invention leaves a contract trail in a block chain by using a simple method while obtaining agreement between relevant parties and also retaining the format of one electronic signature per single transaction. In addition, the present invention maintains reliability while leaving a contract trail in a block chain by using a simple method while obtaining agreement between relevant parties and also retaining the format of one electronic signature per single transaction. The transactions of an issuer side terminal 1 relating to a contract agreement, an agreeing party side terminal 3a and an agreeing party side terminal 3b are chained, and the last agreeing party side terminal 3b generates a transaction directed to the issuer side terminal 1 of the contract issuing source. In addition, the terminals generate, in advance, a key pair for encoding the contract, and send the encoding keys to the terminal of a sender of the transaction. The sender's terminal uses the encoding keys of a receiver of the trans
