In a feedforward amplifier (200) including a signal cancellation circuit (10) and a distortion eliminating circuit (20), a harmonic reaction amplifier (130) is used as a main amplifier of the signal cancellation circuit. A controller (43) obtains an adjacent channel leakage power ratio (ACLR) and a power efficiency from an output of the feedforward amplifier (200) and controls the gate bias voltages of two transistors (33A and 33B) of the harmonic reaction amplifier (130) to maximize the power efficiency under the condition that the ACLR is less than or equal to a reference value.