1. A method of managing a service level agreement (SLA) with respect to devices on a network by using a server, the method comprising: selecting at least one SLA item to quantitatively measure service levels provided by the devices; setting a service management automatic option with respect to the selected SLA item; monitoring the service levels provided by the devices with respect to the selected SLA item; and executing a predetermined event to control the devices according to a result of the monitoring and a setting of the service management automatic option.