An amplifier circuit (10) amplifies a signal inputted from an input terminal (P1). A first feedback circuit (20) is placed across an emitter of a bipolar transistor (101) and an input of the amplifier circuit (10). A second feedback circuit (30) is placed across the input and an output of the amplifier circuit (10) for feeding the output of the amplifier circuit (10) back to the input. A phase change amount in the feedback circuit is determined by the values of an inductor and a capacitor. The values of these elements are selected so that a phase of a signal in which fundamental waves included in two feedback signals are combined and a phase of a signal in which second harmonics included in the two feedback signals are combined are shifted by approximately 180 degrees from a phase of a fundamental wave of an input signal.