Disclosed is a method for advance detection of output power, before arrival at a boundary frequency between an inband and an outband during frequency tracking in a wireless charging apparatus, the wireless charging apparatus including a first resonator resonating at a lower-limit frequency of the inband and a second resonator resonating at an upper-limit frequency of the inband, to adaptively adjust the frequency based on the power values output from the first and second resonators, and to determine a power transmission control value corresponding to the adjusted frequency, thereby enabling frequency control and power control within the inband.