A power wireless transmitter in a magnetic resonance-based wireless power transmission system, the transmitter comprising: a power supply; a variable drive frequency signal generator configured to receive a drive voltage from the power supply and generate a drive frequency signal for wireless power transmission; a switching unit configured to receive the drive voltage from the power supply and to receive the variable drive frequency signal from the variable drive frequency generator and to switch and amplify the variable drive frequency signal; a wireless power transmission unit configured to receive the variable drive frequency signal from the switching unit and wirelessly transmit a power to a wireless power receiving unit using the variable drive frequency signal; a frequency detection unit configured to detect an amplitude of the variable drive frequency signal outputted from the wireless power transmission unit; and a comparator configured to compare the detected amplitude of the variable drive frequency signal to a reference value as a predetermined amplitude, characterized in that the predetermined amplitude is set to a value above a predetermined percentage of a maximum amplitude, wherein the maximum amplitude is an amplitude as measured at an absence of a metal between the wireless power transmission unit and wireless power receiving unit, wherein when the detected amplitude of the variable drive frequency signal is larger than or equal to the reference value as the predetermined amplitude, the frequency of the variable drive frequency signal is determined as a drive frequency for transmission from the wireless power transmission unit, wherein the determined frequency is fed back to the variable drive frequency signal generator, and, thus, the variable drive frequency signal generator generates a drive frequency signal using the fed drive frequency, such that a drive frequency is determined to be applied for a transmission from the power wireless transmitter with a maximum output under a presence of metal. a power supply; a variable drive frequency signal generator configured to receive a drive voltage from the power supply and generate a drive frequency signal for wireless power transmission; a switching unit configured to receive the drive voltage from the power supply and to receive the variable drive frequency signal from the variable drive frequency generator and to switch and amplify the variable drive frequency signal; a wireless power transmission unit configured to receive the variable drive frequency signal from the switching unit and wirelessly transmit a power to a wireless power receiving unit using the variable drive frequency signal; a frequency detection unit configured to detect an amplitude of the variable drive frequency signal outputted from the wireless power transmission unit; and a comparator configured to compare the detected amplitude of the variable drive frequency signal to a reference value as a predetermined amplitude, characterized in that the predetermined amplitude is set to a value above a predetermined percentage of a maximum amplitude, wherein the maximum amplitude is an amplitude as measured at an absence of a metal between the wireless power transmission unit and wireless power receiving unit, wherein when the detected amplitude of the variable drive frequency signal is larger than or equal to the reference value as the predetermined amplitude, the frequency of the variable drive frequency signal is determined as a drive frequency for transmission from the wireless power transmission unit, wherein the determined frequency is fed back to the variable drive frequency signal generator, and, thus, the variable drive frequency signal generator generates a drive frequency signal using the fed drive frequency, such that a drive frequency is determined to be applied for a transmission from the power wireless transmitter with a maximum output under a presence of metal.