A method of determining a magnetic resonant condition for a wireless power transfer system capable of performing a wireless power transfer using at least two receivers to charge the at least two receivers, the method comprising: determining a first charge state for at least one transmitter and the at least two receivers of the wireless power transfer system; and determining a resonant condition with varying at least one design variable in the wireless power transfer system by a preset magnitude in the first charge state; wherein the at least one design variable comprises at least one parameter among an operating frequency, a compensation capacitance, and an input voltage, wherein the resonant condition comprises a value of the at least one design variable that maximizes a power transfer efficiency while satisfying a charge capacity of each of the at least two receivers, and wherein the determining of the resonant condition comprises: detecting a state change from the first charge state to a second charge state, wherein the detecting of the state change comprises detecting a change in at least one of a number of transmitters of the wireless power transfer system, a number of receivers that are supplied with wireless power through the wireless power transfer system, a charge capacity of receivers, a relative position between a transmitter and a receiver, and an orientation angle of a receiver being charged, wherein the change is detected based on an output value of a sensor included in the wireless power transfer system, the sensor being a proximity sensor, a pressure sensor, and/or a motion sensor; and redetermining the resonant condition with varying at least one of an operating frequency, a compensation capacitance, and an input voltage by the preset magnitude as the at least one design variable in the wireless power transfer system, in the second charge state. determining a first charge state for at least one transmitter and the at least two receivers of the wireless power transfer system; and determining a resonant condition with varying at least one design variable in the wireless power transfer system by a preset magnitude in the first charge state; wherein the at least one design variable comprises at least one parameter among an operating frequency, a compensation capacitance, and an input voltage, wherein the resonant condition comprises a value of the at least one design variable that maximizes a power transfer efficiency while satisfying a charge capacity of each of the at least two receivers, and wherein the determining of the resonant condition comprises: detecting a state change from the first charge state to a second charge state, wherein the detecting of the state change comprises detecting a change in at least one of a number of transmitters of the wireless power transfer system, a number of receivers that are supplied with wireless power through the wireless power transfer system, a charge capacity of receivers, a relative position between a transmitter and a receiver, and an orientation angle of a receiver being charged, wherein the change is detected based on an output value of a sensor included in the wireless power transfer system, the sensor being a proximity sensor, a pressure sensor, and/or a motion sensor; and redetermining the resonant condition with varying at least one of an operating frequency, a compensation capacitance, and an input voltage by the preset magnitude as the at least one design variable in the wireless power transfer system, in the second charge state. detecting a state change from the first charge state to a second charge state, wherein the detecting of the state change comprises detecting a change in at least one of a number of transmitters of the wireless power transfer system, a number of receivers that are supplied with wireless power through the wireless power transfer system, a charge capacity of receivers, a relative position between a transmitter and a receiver, and an orientation angle of a receiver being charged, wherein the change is detected based on an output value of a sensor included in the wireless power transfer system, the sensor being a proximity sensor, a pressure sensor, and/or a motion sensor; and redetermining the resonant condition with varying at least one of an operating frequency, a compensation capacitance, and an input voltage by the preset magnitude as the at least one design variable in the wireless power transfer system, in the second charge state.