To provide a method for performing phase control of coherent two light wave signals while maintaining coherence in the state of the light signal, and a device which realizes such a method, there is provided, as illustrated in FIG. 1, a phase adjustment device 1 for two light waves including: a two light wave source 3, a wavelength separator 5, a first phase modulator 7, and a second phase modulator 9, whereby coherent two light waves are used as input signals to perform wavelength separation of those input signals thereafter to control optical phases of the respective light signals thereafter to multiplex them by using a multiplexer 11, thus to be able to obtain an output signal of which optical phase has been adjusted.