When an input signal level is small, the electrical length of a phase line 21 and the electrical length of a phase line 23 are set in such a manner that the impedance seen by looking into the output side from an impedance reference point 11 at the output side of a carrier amplifier 3 becomes 2R+α (where R is a load resistance and α is positive), and the electrical length of a phase line 22 is set at a difference between the electrical length of the phase line 21 and the electrical length of the phase line 23.