An index management device of a service level agreement system includes: an external link processor for receiving a customized service request for requesting a different service per subscriber received by a subscriber subscription management system; a quality manager for extracting needed information from output information from the external link processor; an index management interface for checking a characteristic of the output information from the quality manager and an information management method; a table storage unit for storing index information of the service level agreement system; and an index manager for managing the index information stored in the table storage unit according to the customized service request and a request from the index management interface.