A device-registration-operation assisting apparatus in a device management system including a plurality of devices installed in a building and a management device connected to the devices via a network, assists in an operation to register the devices. The device-registration-operation assisting apparatus includes an emission unit of an electromagnetic wave, and a mobile terminal. The emission unit is integrated into or positioned near one of the plurality of devices. The mobile terminal includes a receiving unit that receives the electromagnetic wave, a display unit that displays information of the device, and an input unit that receives an input of registration information in order to register the device displayed on the display unit in the device management system. The display unit does not display the information of the device for which the registration information is input to the input unit, or changes display of the information.