A microwave semiconductor amplifier with an output terminal connected to an external load, the amplifier having a prescribed frequency band, comprising: a semiconductor amplifying element including an input electrode and an output electrode and having a capacitive output impedance in the frequency band; an input matching circuit connected to the input electrode; and an output matching circuit including a bonding wire and a first transmission line, the bonding wire including a first end portion and a second end portion on an opposite side of the first end portion, the first end portion being connected to the output electrode, the second end portion being connected to one end portion of the first transmission line, the first transmission line being cascaded to the output terminal, the first transmission line having a first characteristic impedance and a first electrical length of 90 degrees or less at an upper limit frequency, the output matching circuit matching the capacitive output impedance of the semiconductor amplifying element to a fundamental impedance of the external load by adding an inductive reactance of the bonding wire to a fundamental impedance seen toward the external load from the one end portion of the first transmission line, and, a second harmonic impedance seen from the output toward the external load from the semiconductor amplifying element being made closer to an open-circuit impedance by adding an inductive reactance of the bonding wire to an inductive second harmonic impedance seen toward the external load from the one end portion of the first transmission line within a frequency range of double a lower limit frequency of the frequency band to double an upper limit frequency of the frequency band. a semiconductor amplifying element including an input electrode and an output electrode and having a capacitive output impedance in the frequency band; an input matching circuit connected to the input electrode; and an output matching circuit including a bonding wire and a first transmission line, the bonding wire including a first end portion and a second end portion on an opposite side of the first end portion, the first end portion being connected to the output electrode, the second end portion being connected to one end portion of the first transmission line, the first transmission line being cascaded to the output terminal, the first transmission line having a first characteristic impedance and a first electrical length of 90 degrees or less at an upper limit frequency, the output matching circuit matching the capacitive output impedance of the semiconductor amplifying element to a fundamental impedance of the external load by adding an inductive reactance of the bonding wire to a fundamental impedance seen toward the external load from the one end portion of the first transmission line, and, a second harmonic impedance seen from the output toward the external load from the semiconductor amplifying element being made closer to an open-circuit impedance by adding an inductive reactance of the bonding wire to an inductive second harmonic impedance seen toward the external load from the one end portion of the first transmission line within a frequency range of double a lower limit frequency of the frequency band to double an upper limit frequency of the frequency band.