1. A system for managing customized service level agreement (SLA) monitoring by being linked with an external system, comprising: a line information management unit that is linked with the external system to receive an external index order for a customer line subscribed in a customized SLA monitoring service and update SLA management data by reflecting demand of the customer included in the external index order on the SLA management data; and a monitoring management unit that performs integrated monitoring on the external system linked with the customer line by using the SLA management data updated by the line information management unit, wherein the monitoring management unit maps a monitoring term for the integrated monitoring by using the SLA management data and performs the integrated monitoring, wherein the line information management unit extracts the monitoring data information from the external index order and reflects contents corresponding to the monitoring data information on a table that is formed corresponding to the SLA management data, and wherein the SLA management data includes customized SLA subscription information, customized index information, an external system monitoring setting flag, a monitoring key identifier, and group setting information.