To provide a bias circuit that can absorb unwanted waves from a low frequency band up to a high frequency band without affecting the amplifier characteristics in a required frequency band.

A FETI is a high frequency amplification transistor(TR), and its output leads to a terminal 5 through a matching circuit 3. A bias circuit 6 is connected to a bias application terminal 7 via a transmission path 8, a λ/4 line 9, a transmission line 14 and a resistor 15. An unwanted wave at a high frequency band generated from the FETI is leaked to the bias circuit 6 and reaches the bias application terminal 7 through the transmission lines 8, 14. Since the length of the transmission line 14 is selected to be a half wavelength with respect to the frequency of the unwanted wave, the unwanted wave across the line 14 go to the resistor 15, where the energy of the unwanted wave is absorbed, resulting that the unwanted wave does not reach the bias application terminal 7. The resistor 15 is connected in parallel with the half wavelength transmission line 14.