A leakage power reduction apparatus for suppressing a leaking transmission signal leaking from a transmitter side to a receiver side, directly or through reflection by an antenna, the apparatus comprising: a circulator which comprises a first port, a second port, and a third port, and is adapted to output a transmission signal input from the first port to the second port, and to output a signal input from the second port to the third port; a duplexer which comprises a first terminal, a second terminal, and a third terminal, the first and the second terminals being connected to the second port of the circulator and the antenna, respectively; an amplitude-and-phase adjuster which is connected to the third port of the circulator and is configured to adjust the amplitude and phase of the signal input from the circulator, and output the adjusted signal as an offset signal for the transmission frequency band; and a combiner which is configured to receive an output signal sent from the third terminal of the duplexer and the offset signal for the transmission frequency band sent from the amplitude-and-phase adjuster, and output a signal obtained by suppressing a transmission frequency band component of the leaking transmission signal included in the output signal sent from the third terminal of the duplexer by the offset signal for the transmission frequency band; characterized in that said duplexer is configured such that a signal in a transmission frequency band is allowed to pass and a signal in a reception frequency band is suppressed between the first terminal and the second terminal, a signal in the reception frequency band being allowed to pass and a signal in the transmission frequency band is suppressed between the second terminal and the third terminal, and signals in the transmission frequency band and the reception frequency band both are suppressed between the first terminal and the third terminal; said amplitude-and-phase adjuster comprises an attenuator for adjusting the amplitude of an input signal; said leakage power reduction apparatus further comprises: a filter which has one end connected between the second port of the circulator and the first terminal of the duplexer, wherein the filter is a reception band-pass filter and is configured to attenuate components outside the reception frequency band, or wherein the filter is a transmission band-elimination filter and is configured to attenuate the transmission frequency band component; and a terminator which is connected to the other end of the filter and is configured to absorb a signal input thereto. a circulator which comprises a first port, a second port, and a third port, and is adapted to output a transmission signal input from the first port to the second port, and to output a signal input from the second port to the third port; a duplexer which comprises a first terminal, a second terminal, and a third terminal, the first and the second terminals being connected to the second port of the circulator and the antenna, respectively; an amplitude-and-phase adjuster which is connected to the third port of the circulator and is configured to adjust the amplitude and phase of the signal input from the circulator, and output the adjusted signal as an offset signal for the transmission frequency band; and a combiner which is configured to receive an output signal sent from the third terminal of the duplexer and the offset signal for the transmission frequency band sent from the amplitude-and-phase adjuster, and output a signal obtained by suppressing a transmission frequency band component of the leaking transmission signal included in the output signal sent from the third terminal of the duplexer by the offset signal for the transmission frequency band; characterized in that said duplexer is configured such that a signal in a transmission frequency band is allowed to pass and a signal in a reception frequency band is suppressed between the first terminal and the second terminal, a signal in the reception frequency band being allowed to pass and a signal in the transmission frequency band is suppressed between the second terminal and the third terminal, and signals in the transmission frequency band and the reception frequency band both are suppressed between the first terminal and the third terminal; said amplitude-and-phase adjuster comprises an attenuator for adjusting the amplitude of an input signal; said leakage power reduction apparatus further comprises: a filter which has one end connected between the second port of the circulator and the first terminal of the duplexer, wherein the filter is a reception band-pass filter and is configured to attenuate components outside the reception frequency band, or wherein the filter is a transmission band-elimination filter and is configured to attenuate the transmission frequency band component; and a terminator which is connected to the other end of the filter and is configured to absorb a signal input thereto. a filter which has one end connected between the second port of the circulator and the first terminal of the duplexer, wherein the filter is a reception band-pass filter and is configured to attenuate components outside the reception frequency band, or wherein the filter is a transmission band-elimination filter and is configured to attenuate the transmission frequency band component; and a terminator which is connected to the other end of the filter and is configured to absorb a signal input thereto.