PURPOSE: To guarantee that a user name to be newly registered is unique in the entire electronic mail system when the user name is unique in a user name dividing area in the case of newly registering the user name by hierarchically managing users in the electronic mail system.

CONSTITUTION: A user name dividing area managing means 5 investigates whether a user name constituting attribute value 22 of the user name to be registered is already registered to a most-significant user name constituting attribute 21 or not, and when the attribute value 22 is already registered, the information of a user name dividing area 23 corresponding to the user name constituting attribute value 22 is possessed. A user name in user name dividing area managing means 6 investigates whether the user name constituting attribute value 22 of the user name to be registered is already registered to the user name constituting attribute 21 at the next level or not. When the attribute value 22 is not registered, the user name having the user name constituting attribute value 22 does not exist in the user name dividing area 23. Thus, it is guaranteed that the user name is unique even in an entire electronic mail system 1.