To enable the safe preservation of transaction information in an electronic commercial transaction on a network by preparing a session key for enciphering the transaction information, enciphering the transaction information while using the session key, and sending the enciphered transaction information to a preserving server on the network.

A session key 620 is prepared for enciphering a transaction log 610, and a transaction log 615 is provided by enciphering the transaction log 610 while using this session key. Next, key recovery information 640 for recovering this session key 620 is prepared while using key recovery techniques. Further, the session key 620 is enciphered while using a secret key 650 in a master card 600 and a session key 625 is provided. Then, these three, namely, the enciphered transaction log 615, the key recovery information 640 for recovering the session key 620 and the enciphered session key 625 are sent to a preserving server 380.