A travelling wave power combiner ensures high isolation between power amplifiers and reduces harmonic components caused by non-linearity of each power amplifier. Therefore, the travelling wave power circuit is provided with even-numbered power amplifiers, a plurality of series-connected branch circuits for respectively distributing input power to the even-numbered power amplifiers, and a plurality of series-connected combiners for combining respective output power of the even-numbered power amplifiers. With respect to all of such combinations as to equalize electric lengths of transmission lines for connecting between the power amplifiers to one another, which combinations are established for the plurality of power amplifiers, the electric lengths at which the combinations are established, take &lgr;/2i (where &lgr;=the wavelength of a fundamental wave, and i=positive integer).