A high-frequency power amplifier (100, 100a, 100b, 100c) including an input terminal, an input matching circuit (110, 110a), an amplifying element, an output matching circuit (130, 130a), a coupling circuit (140, 140b), a detection circuit, and an output terminal, wherein either the input matching circuit or the output matching circuit has an active element (160, 160a), the input matching circuit (110, 110a) is configured to receive a signal from the input terminal and output the signal to the amplifying element, the amplifying element is configured to receive and amplify the signal outputted by the input matching circuit (110, 110a), and output the amplified signal to the output matching circuit (130, 130a), the output matching circuit (130, 130a) is configured to receive the signal outputted by the amplifying element and output the signal to the coupling circuit (140, 140b), the coupling circuit (140, 140b) is configured to receive and divide the signal outputted by the output matching circuit (130, 130a) into signals, and output the signals to the detection circuit and the output terminal, respectively, the detection circuit is configured to receive one of the signals outputted by the coupling circuit (140, 140b), convert the one of the signals into a voltage, and output the voltage to the active element (160, 160a) as a control voltage, and the active element (160, 160a) is configured to receive the control voltage outputted by the detection circuit and change impedance of the active element (160, 160a) in accordance with the control voltage, thereby changing power exhibited by the signal outputted by either the input matching circuit having the active element or the output matching circuit having the active element, to change power of a signal which the coupling circuit (140, 140b) outputs to the output terminal. either the input matching circuit or the output matching circuit has an active element (160, 160a), the input matching circuit (110, 110a) is configured to receive a signal from the input terminal and output the signal to the amplifying element, the amplifying element is configured to receive and amplify the signal outputted by the input matching circuit (110, 110a), and output the amplified signal to the output matching circuit (130, 130a), the output matching circuit (130, 130a) is configured to receive the signal outputted by the amplifying element and output the signal to the coupling circuit (140, 140b), the coupling circuit (140, 140b) is configured to receive and divide the signal outputted by the output matching circuit (130, 130a) into signals, and output the signals to the detection circuit and the output terminal, respectively, the detection circuit is configured to receive one of the signals outputted by the coupling circuit (140, 140b), convert the one of the signals into a voltage, and output the voltage to the active element (160, 160a) as a control voltage, and the active element (160, 160a) is configured to receive the control voltage outputted by the detection circuit and change impedance of the active element (160, 160a) in accordance with the control voltage, thereby changing power exhibited by the signal outputted by either the input matching circuit having the active element or the output matching circuit having the active element, to change power of a signal which the coupling circuit (140, 140b) outputs to the output terminal.