An amplifier comprising: a pair of input terminals (101A, 101B) configured to receive as input a transmission signal being differential and two-tone; a pair of output terminals (102A, 102B); a coil configured to have both terminals connected with the pair of input terminals, respectively, and to have a center tap; a first transistor configured to have a gate connected with one of the terminals of the coil, and an output terminal of the first transistor connected with one of the pair of output terminals; a second transistor configured to have a gate connected with another of the terminals of the coil, and an output terminal of the second transistor connected with another of the pair of output terminals; a diode configured to have a terminal connected with the center tap of the coil; and a gate voltage generation unit configured to be connected with another terminal of the diode, and to output a predetermined gate voltage to turn on the first transistor and the second transistor; and a capacitor configured to have a terminal connected with a point between the other terminal of the diode and a terminal of the gate voltage generation unit, and another terminal connected with a reference potential point, wherein an inductance of the coil and a capacitance of the capacitor are set to respective values to configure an LC resonance circuit with a resonance frequency set to the frequency of a double harmonic wave of the transmission signal input into the pair of input terminals (101A, 101B); and the diode is configured to adjust a voltage at the terminal of the diode depending on a signal level of the double harmonic wave of the transmission signal that is supplied to the terminal of the diode from the center tap of the coil. a pair of input terminals (101A, 101B) configured to receive as input a transmission signal being differential and two-tone; a pair of output terminals (102A, 102B); a coil configured to have both terminals connected with the pair of input terminals, respectively, and to have a center tap; a first transistor configured to have a gate connected with one of the terminals of the coil, and an output terminal of the first transistor connected with one of the pair of output terminals; a second transistor configured to have a gate connected with another of the terminals of the coil, and an output terminal of the second transistor connected with another of the pair of output terminals; a diode configured to have a terminal connected with the center tap of the coil; and a gate voltage generation unit configured to be connected with another terminal of the diode, and to output a predetermined gate voltage to turn on the first transistor and the second transistor; and a capacitor configured to have a terminal connected with a point between the other terminal of the diode and a terminal of the gate voltage generation unit, and another terminal connected with a reference potential point, wherein an inductance of the coil and a capacitance of the capacitor are set to respective values to configure an LC resonance circuit with a resonance frequency set to the frequency of a double harmonic wave of the transmission signal input into the pair of input terminals (101A, 101B); and the diode is configured to adjust a voltage at the terminal of the diode depending on a signal level of the double harmonic wave of the transmission signal that is supplied to the terminal of the diode from the center tap of the coil.