A key exchange system comprising a first terminal and a second terminal connected to each other via a network in which a blockchain is constructed, wherein the first terminal comprises: a license issuing unit configured to generate a first transaction including a first public key and an issued license responding to a license request; a first key storage region configured to retain a first secret key; and a first encrypting and decrypting unit configured to generate a common key using deal-specific information, including the first transaction, a second transaction, and a parameter of a specific block, together with a second public key and the first secret key, the second terminal comprises: a license requesting unit configured to generate the second transaction including the second public key and the license request; a second key storage region configured to retain a second secret key; and a second encrypting and decrypting unit configured to generate the common key using the deal-specific information, the first public key, and the second secret key, the first encrypting and decrypting unit is configured to encrypt secret information using the common key generated by the first encrypting and decrypting unit, and the second encrypting and decrypting unit is configured to decrypt the encrypted secret information acquired via the network using the common key generated by the second encrypting and decrypting unit, wherein the parameter of the specific block is a unique number allocated to a block in which the first transaction or the second transaction is stored. a license issuing unit configured to generate a first transaction including a first public key and an issued license responding to a license request; a first key storage region configured to retain a first secret key; and a first encrypting and decrypting unit configured to generate a common key using deal-specific information, including the first transaction, a second transaction, and a parameter of a specific block, together with a second public key and the first secret key, the second terminal comprises: a license requesting unit configured to generate the second transaction including the second public key and the license request; a second key storage region configured to retain a second secret key; and a second encrypting and decrypting unit configured to generate the common key using the deal-specific information, the first public key, and the second secret key, the first encrypting and decrypting unit is configured to encrypt secret information using the common key generated by the first encrypting and decrypting unit, and the second encrypting and decrypting unit is configured to decrypt the encrypted secret information acquired via the network using the common key generated by the second encrypting and decrypting unit, wherein the parameter of the specific block is a unique number allocated to a block in which the first transaction or the second transaction is stored. a license requesting unit configured to generate the second transaction including the second public key and the license request; a second key storage region configured to retain a second secret key; and a second encrypting and decrypting unit configured to generate the common key using the deal-specific information, the first public key, and the second secret key, the first encrypting and decrypting unit is configured to encrypt secret information using the common key generated by the first encrypting and decrypting unit, and the second encrypting and decrypting unit is configured to decrypt the encrypted secret information acquired via the network using the common key generated by the second encrypting and decrypting unit, wherein the parameter of the specific block is a unique number allocated to a block in which the first transaction or the second transaction is stored.