A semiconductor amplifier comprising: a semiconductor amplifying element including an input electrode and an output electrode; an output terminal connected with an external load; an output matching circuit including a bonding wire, a first transmission line having a first characteristic impedance and a first electrical length, and a second transmission line having a second characteristic impedance and a second electrical length, the bonding wire including one end part connected to the output electrode and the other end part connected to one end part of the first transmission line, the other end part of the first transmission line being connected to one end part of the second transmission line, the other end part of the second transmission line being connected to the output terminal, the first electrical length being 90° or less at an upper limit frequency of a prescribed band, the second characteristic impedance being larger than the first characteristic impedance; and an output bias circuit including a third transmission line having a third characteristic impedance and an electrical length of not less than 81° and not more than 99° at a center frequency in the prescribed band, a grounded shunt capacitor, and a power supply terminal, the third transmission line including one end part and the other end part connected to the grounded shunt capacitor, the one end part of the third transmission line being connected to the second transmission line at a position where an electrical length is not less than 40.5° and not more than 49.5° from the one end part of the second transmission line toward the output terminal at the center frequency, the power supply terminal being connected to the other end part of the third transmission line, thereby achieving a load impedance seen from a reference plane, when looking towards the output, between the first transmission line and the second transmission line that is open at a second harmonic center frequency, and a load impedance at the central frequency seen from the reference plane looking towards the output that is configured to match with a capacitive output impedance of the semiconductor amplifying element via the bonding wire and the first transmission line in the prescribed band. a semiconductor amplifying element including an input electrode and an output electrode; an output terminal connected with an external load; an output matching circuit including a bonding wire, a first transmission line having a first characteristic impedance and a first electrical length, and a second transmission line having a second characteristic impedance and a second electrical length, the bonding wire including one end part connected to the output electrode and the other end part connected to one end part of the first transmission line, the other end part of the first transmission line being connected to one end part of the second transmission line, the other end part of the second transmission line being connected to the output terminal, the first electrical length being 90° or less at an upper limit frequency of a prescribed band, the second characteristic impedance being larger than the first characteristic impedance; and an output bias circuit including a third transmission line having a third characteristic impedance and an electrical length of not less than 81° and not more than 99° at a center frequency in the prescribed band, a grounded shunt capacitor, and a power supply terminal, the third transmission line including one end part and the other end part connected to the grounded shunt capacitor, the one end part of the third transmission line being connected to the second transmission line at a position where an electrical length is not less than 40.5° and not more than 49.5° from the one end part of the second transmission line toward the output terminal at the center frequency, the power supply terminal being connected to the other end part of the third transmission line, thereby achieving a load impedance seen from a reference plane, when looking towards the output, between the first transmission line and the second transmission line that is open at a second harmonic center frequency, and a load impedance at the central frequency seen from the reference plane looking towards the output that is configured to match with a capacitive output impedance of the semiconductor amplifying element via the bonding wire and the first transmission line in the prescribed band.