In order to give sufficient evidential admissibility to the content of a contract made online, contractual terms data C that shows a character string of contractual terms is prepared and stored by a processor on the side of a transacting party A. The data C is then transmitted to a processor on the side of a transacting party B, and is stored. A hash value ID2 of the contractual terms data C is calculated by the B-side processor, and a signature Sig2 of the party B is added and transmitted to the A-side processor as formal contractual proposal information. The signature Sig2 is verified by the A-side processor. A coincidence is verified between a hash value ID1 of the data C and the ID2 that has been transmitted. A signature Sig1 of the party A is added to a coinciding hash value ID, and is transmitted to the B-side processor as formal contractual acceptance information. The signature Sig1 is verified by the B-side processor, and a coincidence is verified between the hash value ID2 and the ID that has been transmitted.