PURPOSE: To avoid such an inconvenient case where a password is easily used by an outsider by making a user use a character string evolved from another character string registered previously as a password and therefore using the different character strings as passwords for each input of passwords.

CONSTITUTION: In a computer system the character strings having the finite lengths are registered into a password control information storing means 30 as the password register character strings 31 for each user. The means 30 stores the password register character string access control information 32 as the control information which have accesses to the strings 31. The information 32 is used as an index to show an optional position in the string 31 and initialized when the string 31 is registered. Then a user can have an access to the computer system when the registration of a password is through. When the user inputs a password, a character string length extracting means 10 obtains the character string length of the password. This character string length is sent to a password control information access means 20.