Provided is a network system including an advanced metering infrastructure, an energy management system, a computer, and a diagnostic program. The advanced metering infrastructure interactively communicates with a power supply source and measures energy from the power supply source. The energy management system is connected to the advanced metering infrastructure to communicate with it and controls an operation of an electric product based on operation information of the electric product or energy information from the power supply source. The computer is connected to the electric product or energy management system to communicate with the electric product or energy management system. The diagnostic program is performed by the energy management system or computer and diagnoses a trouble of the electric product. An electric product having a trouble is remotely diagnosed to provide a quick countermeasure, thereby detecting the cause of the trouble without a service engineer and improving a user's convenience.