To realize a power synthesizing device with a simple configuration capable of improving isolation characteristics in a state where a plurality of transmission lines receiving inputted power have the same impedance and a phase difference is 90.

This power synthesizing device has a distribution coupling type directional coupler consisting of a main line and a sub-line electromagnetically coupled to the main line; first and second transmission lines connected to both ends of the main line; and third and fourth transmission lines connected to both ends of the sub-line. The device synthesizes the input power from the second transmission line connected to the main line and the third transmission line connected to the sub-line to the first transmission line connected to the main line, wherein a 1/4 wavelength transformer having characteristic impedance different from that of the first transmission line is inserted into the first transmission line connected to the main line.