An amplifier comprising: a first power amplifier (101, 101A) configured to amplify an unmodulated high-frequency input generated by a microwave signal source; a first branching device which is provided at an input terminal of the first power amplifier (101, 101A) and is adapted to extract the input signal; a second branching device which is provided at an output terminal of the first power amplifier (101,101A) and is adapted to extract the output signal; a mixer configured to mix the signal from the first branching device and the signal from the second branching device; characterized in that if an abnormal oscillation occurs near the frequency band of the input signal, a voltage amplitude of the output signal of the first branching device has at least one frequency component of other than the input signal; and in that the amplifier further comprises: a low-pass filter configured to remove a frequency band component of the input signal from the output signal of the mixer; a high-pass filter configured to remove a DC component from the output signal of the low-pass filter; and a wave detector configured to extract a signal which has a frequency component with a frequency which is the difference between the frequency of the input signal and the frequency other than the frequency of the input signal; and a frequency component with a frequency that is the sum of the frequency of the input signal and the frequency other than the frequency of the input signal. a first power amplifier (101, 101A) configured to amplify an unmodulated high-frequency input generated by a microwave signal source; a first branching device which is provided at an input terminal of the first power amplifier (101, 101A) and is adapted to extract the input signal; a second branching device which is provided at an output terminal of the first power amplifier (101,101A) and is adapted to extract the output signal; a mixer configured to mix the signal from the first branching device and the signal from the second branching device; characterized in that if an abnormal oscillation occurs near the frequency band of the input signal, a voltage amplitude of the output signal of the first branching device has at least one frequency component of other than the input signal; and in that the amplifier further comprises: a low-pass filter configured to remove a frequency band component of the input signal from the output signal of the mixer; a high-pass filter configured to remove a DC component from the output signal of the low-pass filter; and a wave detector configured to extract a signal which has a frequency component with a frequency which is the difference between the frequency of the input signal and the frequency other than the frequency of the input signal; and a frequency component with a frequency that is the sum of the frequency of the input signal and the frequency other than the frequency of the input signal. a low-pass filter configured to remove a frequency band component of the input signal from the output signal of the mixer; a high-pass filter configured to remove a DC component from the output signal of the low-pass filter; and a wave detector configured to extract a signal which has a frequency component with a frequency which is the difference between the frequency of the input signal and the frequency other than the frequency of the input signal; and a frequency component with a frequency that is the sum of the frequency of the input signal and the frequency other than the frequency of the input signal.