A database system is provided with a search unit for searching image attribute information that has been stored in a relational database management system. A search interface is provided between the search unit and a command execution unit, which applies a search command to the search unit. The command execution unit and search unit can be connected in detachable fashion. Thus a search unit that has been connected to the command execution unit can be replaced by detaching it and connecting a search unit that employs another search technique.