A power divider/combiner, comprising: a common terminal (9001, 9101), to which a high-frequency signal to be divided is inputtable, or from which a synthesized high-frequency signal is outputtable; a first input/output terminal (9002, 9102) and a second input/output terminal (9003, 9103), from which divided high-frequency signals are outputtable, or to which high-frequency signals to be synthesized are inputtable; a first impedance transformer (1020, 1120) having one end connected to the common terminal (9001, 9101) and another end connected to the first input/output terminal (9002, 9102); a second impedance transformer (1030, 1130) having one end connected to the common terminal (9001, 9101) and another end connected to the second input/output terminal (9003, 9103); an isolation resistor configured to avoid interference between a high-frequency signal at the first input/output terminal (9002, 9102) and a high-frequency signal at the second input/output terminal (9003, 9103); a first transmission line and a second transmission line, which are cascaded and wherein a first end of the cascaded first transmission line and second transmission line is connected to a first end of the isolation resistor and wherein a second end of the cascaded first transmission line and second transmission line is connected to the first input/output terminal (9002, 9102); and a third transmission line and a fourth transmission line, which are cascaded and wherein a first end of the cascaded third transmission line and fourth transmission line is connected to a second end of the isolation resistor and wherein a second end of the cascaded third transmission line and fourth transmission line is connected to the second input/output terminal (9003, 9103), the first transmission line being connected to the first input/output terminal (9002, 9102) and the third transmission line being connected to the second input/output terminal (9003, 9103), and the first transmission line and the third transmission line being arranged in parallel and close to each other, and also being electrically coupled to form a first coupling line. a common terminal (9001, 9101), to which a high-frequency signal to be divided is inputtable, or from which a synthesized high-frequency signal is outputtable; a first input/output terminal (9002, 9102) and a second input/output terminal (9003, 9103), from which divided high-frequency signals are outputtable, or to which high-frequency signals to be synthesized are inputtable; a first impedance transformer (1020, 1120) having one end connected to the common terminal (9001, 9101) and another end connected to the first input/output terminal (9002, 9102); a second impedance transformer (1030, 1130) having one end connected to the common terminal (9001, 9101) and another end connected to the second input/output terminal (9003, 9103); an isolation resistor configured to avoid interference between a high-frequency signal at the first input/output terminal (9002, 9102) and a high-frequency signal at the second input/output terminal (9003, 9103); a first transmission line and a second transmission line, which are cascaded and wherein a first end of the cascaded first transmission line and second transmission line is connected to a first end of the isolation resistor and wherein a second end of the cascaded first transmission line and second transmission line is connected to the first input/output terminal (9002, 9102); and a third transmission line and a fourth transmission line, which are cascaded and wherein a first end of the cascaded third transmission line and fourth transmission line is connected to a second end of the isolation resistor and wherein a second end of the cascaded third transmission line and fourth transmission line is connected to the second input/output terminal (9003, 9103), the first transmission line being connected to the first input/output terminal (9002, 9102) and the third transmission line being connected to the second input/output terminal (9003, 9103), and the first transmission line and the third transmission line being arranged in parallel and close to each other, and also being electrically coupled to form a first coupling line.