provided is an optical phase modulating method and apparatus for a quantum key distribution . when an optical phase modulator is arranged outside an optical interferometer , a configuration of the optical interferometer may be simplified , and an extension of an optical path caused by the optical phase modulator , instability and an insertion loss increased in the optical interferometer , and the like , may be overcome . an output feature may be improved by adjusting an applied voltage of the optical phase modulator arranged outside the optical interferometer .