A semiconductor amplifier bias circuit provided between an output matching circuit of a semiconductor amplifying element and an external load comprising: a first transmission line having an input end part and an output end part, the input end part being connected to the output matching circuit, the output end part being connected to the external load; a second transmission line having an electrical length of not less than 81° and not more than 99° at a center frequency of a band, the second transmission line including a one end part connected to the first transmission line, and an other end part; a circuit element for grounding a fundamental signal including one of a first grounded shunt capacitor and an open stub line having an electrical length of not less than 81° and not more than 99° at the center frequency; and a power supply terminal connected to the other end part of the second transmission line, wherein in case of the circuit element including the first grounded shunt capacitor, the one end part of the second transmission line is connected to the first transmission line at a position apart from the input end part of the first transmission line by an electrical length of not less than 40.5° and not more than 49.5° at the center frequency, and the other end part of the second transmission line is connected to the first grounded shunt capacitor, a load impedance seen from the position of the first transmission line is kept at a short-circuited impedance at double the center frequency, and a load impedance seen from the input end part of the first transmission line is transformed to an open-circuited impedance along the first transmission line at double the center frequency, and in case of the circuit element including the open stub line, the open stub line includes one end part connected to the other end part of the second transmission line, and other end part made open, and a load impedance seen from the input end part of the first transmission line being equal to the resistance of the external load or more at double the center frequency. a first transmission line having an input end part and an output end part, the input end part being connected to the output matching circuit, the output end part being connected to the external load; a second transmission line having an electrical length of not less than 81° and not more than 99° at a center frequency of a band, the second transmission line including a one end part connected to the first transmission line, and an other end part; a circuit element for grounding a fundamental signal including one of a first grounded shunt capacitor and an open stub line having an electrical length of not less than 81° and not more than 99° at the center frequency; and a power supply terminal connected to the other end part of the second transmission line, wherein in case of the circuit element including the first grounded shunt capacitor, the one end part of the second transmission line is connected to the first transmission line at a position apart from the input end part of the first transmission line by an electrical length of not less than 40.5° and not more than 49.5° at the center frequency, and the other end part of the second transmission line is connected to the first grounded shunt capacitor, a load impedance seen from the position of the first transmission line is kept at a short-circuited impedance at double the center frequency, and a load impedance seen from the input end part of the first transmission line is transformed to an open-circuited impedance along the first transmission line at double the center frequency, and in case of the circuit element including the open stub line, the open stub line includes one end part connected to the other end part of the second transmission line, and other end part made open, and a load impedance seen from the input end part of the first transmission line being equal to the resistance of the external load or more at double the center frequency.