A demultiplexer/multiplexer includes an input terminal, which receives input signals from respective phases of a quadrifilar helix antenna; phase shifter/separator/mixers, which alternately phase-shift right-handed and left-handed circularly polarized waves of the input signals, respectively, by 90°/−90° to produce phase-shifted waves to be combined in an inphase combination; phase shifter/mixers, which receive the left-handed or right-handed circularly polarized waves from the phase shifter/separator/mixers, and phase-shift one of the left-handed and right-handed circularly polarized waves by 180°/−180° to produce a phase-shifted wave to be combined with the other of the left-handed and right-handed circularly polarized waves in an antiphase combination; a variable phase shifter, which adjusts an output signal from the phase shifter/mixers by a phase-shift amount that is received in advance; and an output terminal, which outputs an output signal from the variable phase shifter and the other output signal that is not input to the variable phase shifter.