According to one embodiment, a power detector includes a reference voltage generator, a square signal generator, a detection circuit, and an output circuit. The reference voltage generator is configured to receive a bias voltage and generate a reference voltage. The square signal generator is configured to receive a voltage having a high frequency input voltage superimposed on the bias voltage and output a signal including the reference voltage, a voltage of a square of the high frequency input voltage, and a high-frequency signal. The detection circuit has a first lowpass filter, a first operational amplifier configured to amplify error between an output voltage of the first lowpass filter and the reference voltage and output the error as a control voltage, and a feedback transistor configured to feed a feedback current according to the control voltage back to an output terminal of the square signal generator.