A field effect transistor amplifer has a high gain in a plurality of microwave frequency bands. In a field effect transistor with the gate terminal and the drain terminal thereof connected to a matching circuit on the input side and a matching circuit on the output side, respectively, a resonance circuit which is composed of a series circuit including at least one second inductor and a capacitor and connected in parallel to a resistor, is connected between at least one of the gate terminal of the field effect transistor and the ground and between the drain terminal of the field effect transistor and the ground. When a series circuit including the first inductor, at least one second inductor and the capacitor is resonated and short-circuited, the gain at the resonance frequency is dropped and a gain in the range outside of the desired bands is suppressed. When a series circuit including at least one second inductor and the capacitor and connected to the resistor is resonated and short-circuited, the drop of the gain at the resonance frequency due to the resistor is suppressed. This thereby enables high gains to be obtained in the desired bands.