To provide a directional coupler branching unit capable of variably controlling a phase difference between two branching high-frequency signals.

The directional coupler branching unit is provided with microstrip lines 1, 2 as first and second high frequency transmission lines which are jointed or located close to each other, in a way such that the midways of the lines are electromagnetically coupled; and a tunable capacitor 3 being a variable impedance element as a variable phase reflector for reflecting an incident high-frequency signal, while changing the phase and located at one end 2a of the second high-frequency transmission line 2 on the same side of an output terminal 1b of the microstrip line 1 as the first high-frequency transmission line, as viewed from the microstrip line 2 as the second high-frequency transmission line. Adjusting the capacitance of the tunable capacitor 3 located at the one end 2a changes the phase of the high-frequency signal reflected in the tunable capacitor 3, thereby enabling variable control of the phase difference between the branched two high-frequency signals.

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