A bias circuit is provided for attenuating harmonic distortions of a signal having a simple construction and which can be applied to a high-frequency amplifier used in a communication device, such as a mobile telephone. The circuit reduces a voltage drop therein and thus provides a high-frequency amplifier having reduced power consumption and an increased operating efficiency. The high-frequency amplifier includes an amplifier circuit, an output matching circuit, and the bias circuit. In the bias circuit, a parallel circuit including a first transmission line and a first capacitor has one end connected between the amplifier and the output matching circuit. The other end of the parallel circuit is connected to a power source and is grounded via a second capacitor. In the circuit, the bias circuit can be short-circuited in a desired frequency band while being an open circuit in a frequency band of a signal to be amplified, hence attenuating the harmonic distortions without using a low pass filter.