In a method of phase adjustment for the demodulator 1 of the present invention, the phase adjustment is performed by driving any one of the heaters on the two waveguides 10 and 11 in the Mach-Zehnder interferometer (MZI) 6 and on the two waveguides 14 and 15 in the MZI 7. In case that an initial phase difference between the MZIs 6 and 7 smaller than a required phase difference as .pi./2 therebetween, the heaters C and D are driven, that are formed on the first waveguide 10 in the MZI 6, and the heaters G and H are driven, that are formed on the second waveguide 15 in the MZI 7. In case that the initial phase difference is larger than the required phase difference (.pi./2) therebetween, the heaters A and B formed on the second waveguide 11 in the MZI 6, and the heaters E and F formed on the first waveguide 14 in the MZI 7 are driven.