PROBLEM TO BE SOLVED : To identify a real estate agent who enters a house.SOLUTION : A lock device 71 comprises an input key 511, a liquid crystal part 512, control means including password generation means which generates a password, and encryption means which encrypts the password. Further, a server 9 includes decryption means which decrypts a cipher. The lock device then comprises a first step for starting the lock device 71 when a real estate agent uses the input key 511, a second step for generating the password by the password generation means, a third step for displaying a cipher encrypted by the encryption means on the liquid crystal part 512, a fourth step for transmitting the cipher to the server 9 by the real estate agent, a fifth step for decrypting the cipher through the decryption means of the server 9, converting it into the password and notifying the real estate agent of the password, and a sixth step for inputting the password to the input key 511 by the real estate agent to unlock the lock device 71.COPYRIGHT : (C)2013, JPO&INPIT