A power divider/combiner, comprising: a common terminal , 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 and a second input/output terminal , from which divided high-frequency signals are outputtable, or to which high-frequency signals to be synthesized are inputtable; a first impedance transformer having one end connected to the common terminal and another end connected to the first input/output terminal ; a second impedance transformer having one end connected to the common terminal and another end connected to the second input/output terminal ; an isolation resistor configured to avoid interference between a high-frequency signal at the first input/output terminal and a high-frequency signal at the second input/output terminal ; 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 ; 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 , the first transmission line being connected to the first input/output terminal and the third transmission line being connected to the second input/output terminal , 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 , 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 and a second input/output terminal , from which divided high-frequency signals are outputtable, or to which high-frequency signals to be synthesized are inputtable; a first impedance transformer having one end connected to the common terminal and another end connected to the first input/output terminal ; a second impedance transformer having one end connected to the common terminal and another end connected to the second input/output terminal ; an isolation resistor configured to avoid interference between a high-frequency signal at the first input/output terminal and a high-frequency signal at the second input/output terminal ; 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 ; 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 , the first transmission line being connected to the first input/output terminal and the third transmission line being connected to the second input/output terminal , 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. <EOS>