source device 110 information about the amount of the power received by the target resonator in this example the information about the amount of the power received by the target resonator may correspond to an input voltage value and an input current value of the rectifier 122 an output voltage value and an output current value of the rectifier 122 or an output voltage value and an output current value of the dc dc converter 123 the controller 114 of the source device 110 may set a resonance bandwidth of the source resonator 131 based on the set resonance bandwidth of the source resonator 131 a q factor q s of the source resonator 131 may be determined the controller 125 may set a resonance bandwidth of the target resonator 133 based on the set resonance bandwidth of the target resonator 133 a q factor q d of the target resonator 133 may be determined in this example the resonance bandwidth of the source resonator 131 may be set to be wider or narrower than the resonance bandwidth of the target resonator 133 by communicating with each other the source device 110 and the target device 120 may share information regarding the resonance bandwidths of the source resonator 131 and the target resonator 133 when a power higher than a reference value is requested by the target device 120 the q factor q s of the source resonator 131 may be set to a value greater than 100 when a power lower than the reference value is requested by the target device 120 the q factor q s of the source resonator 131 may be set to a value less than 100 in resonance based wireless power.