A wireless charging method, by a wireless charging apparatus, comprising: wirelessly transmitting power to an external power receiving device by connecting a power transmitter unit of the wireless charging apparatus to a resonator of the wireless charging apparatus, based on a wireless power transmission mode; obtaining a wireless power reception mode selection input by the wireless charging apparatus; switching from the wireless power transmission mode to the wireless power reception mode based on the obtained wireless power reception mode selection input while operating in the wireless power transmission mode; wirelessly receiving power from an external power transmitting device by connecting a power receiver unit of the wireless charging apparatus to the resonator of the wireless charging apparatus based on the wireless reception mode; obtaining a wireless power relay mode selection input; switching from the wireless power transmission mode to a wireless power relay mode based on the obtained wireless power relay mode selection input while operating in the wireless power transmission mode; and wirelessly relaying power from the external power transmitting device to the external power receiving device by disconnecting both of the power receiver unit and the power transmitter unit from the resonator of the wireless charging apparatus based on the wireless power relay mode, wherein wirelessly transmitting the power, comprises: selecting one of an external power source and a battery of the wireless charging apparatus, when the wireless power transmission mode selected; receiving electric power from the external power source and wirelessly transmitting the power using the electric power from the external power source, when the external power source is selected; and wirelessly transmitting the power using electric power from the battery, when the battery is selected. wirelessly transmitting power to an external power receiving device by connecting a power transmitter unit of the wireless charging apparatus to a resonator of the wireless charging apparatus, based on a wireless power transmission mode; obtaining a wireless power reception mode selection input by the wireless charging apparatus; switching from the wireless power transmission mode to the wireless power reception mode based on the obtained wireless power reception mode selection input while operating in the wireless power transmission mode; wirelessly receiving power from an external power transmitting device by connecting a power receiver unit of the wireless charging apparatus to the resonator of the wireless charging apparatus based on the wireless reception mode; obtaining a wireless power relay mode selection input; switching from the wireless power transmission mode to a wireless power relay mode based on the obtained wireless power relay mode selection input while operating in the wireless power transmission mode; and wirelessly relaying power from the external power transmitting device to the external power receiving device by disconnecting both of the power receiver unit and the power transmitter unit from the resonator of the wireless charging apparatus based on the wireless power relay mode, wherein wirelessly transmitting the power, comprises: selecting one of an external power source and a battery of the wireless charging apparatus, when the wireless power transmission mode selected; receiving electric power from the external power source and wirelessly transmitting the power using the electric power from the external power source, when the external power source is selected; and wirelessly transmitting the power using electric power from the battery, when the battery is selected. selecting one of an external power source and a battery of the wireless charging apparatus, when the wireless power transmission mode selected; receiving electric power from the external power source and wirelessly transmitting the power using the electric power from the external power source, when the external power source is selected; and wirelessly transmitting the power using electric power from the battery, when the battery is selected.