An optical switch comprising: a Mach-Zehnder interferometer having a 3-dB coupler placed on an input side, a 3-dB coupler placed on an output side, and two three-dimensional optical waveguides connecting said input-side 3-dB coupler and said output-side 3-dB coupler together; and a phase modulating part including means for applying electric fields to one or both of said two three-dimensional optical waveguides; wherein said means for applying electric fields to one or both of said two-dimensional optical waveguides has a first electrode structure (22, 23) in a first phase modulation section of the phase modulation part that is adapted to form a first electric field in the thickness direction of the optical waveguides parallel to the TE direction of a propagating light guided in the waveguides and a second electrode structure (22, 23) in a second phase modulation section of the phase modulation part that is adapted to form a second electric field in the width direction of the optical waveguides parallel to the TM direction of the propagating light guided in the waveguides, wherein said means are arranged to apply the first and second electric fields with the same electric field strengths, and characterized in that the cores of the two three-dimensional optical waveguides are formed of a crystal material including KTa 1-x Nb x O 3 (0<x<1) and K 1-y Li y Ta 1-x Nb x O 3 (0<x<1, 0<y<1), or KTa 1-x Nb x O 3 (0<x<1), or K 1-y Li y Ta 1-x Nb x O 3 (0<x<1, 0<y<1), and in that the switch further comprises a temperature controller arranged to adjust the operating temperature of said KTa 1-x Nb x O 3 or K 1-y Li y Ta 1-x Nb x O 3 to the phase transition temperature at which said KTa 1-x Nb x O 3 or K 1-y Li y Ta 1-x Nb x O 3 changes from a tetragonal crystal to a cubic crystal. a Mach-Zehnder interferometer having a 3-dB coupler placed on an input side, a 3-dB coupler placed on an output side, and two three-dimensional optical waveguides connecting said input-side 3-dB coupler and said output-side 3-dB coupler together; and a phase modulating part including means for applying electric fields to one or both of said two three-dimensional optical waveguides; wherein said means for applying electric fields to one or both of said two-dimensional optical waveguides has a first electrode structure (22, 23) in a first phase modulation section of the phase modulation part that is adapted to form a first electric field in the thickness direction of the optical waveguides parallel to the TE direction of a propagating light guided in the waveguides and a second electrode structure (22, 23) in a second phase modulation section of the phase modulation part that is adapted to form a second electric field in the width direction of the optical waveguides parallel to the TM direction of the propagating light guided in the waveguides, wherein said means are arranged to apply the first and second electric fields with the same electric field strengths, and characterized in that the cores of the two three-dimensional optical waveguides are formed of a crystal material including KTa 1-x Nb x O 3 (0<x<1) and K 1-y Li y Ta 1-x Nb x O 3 (0<x<1, 0<y<1), or KTa 1-x Nb x O 3 (0<x<1), or K 1-y Li y Ta 1-x Nb x O 3 (0<x<1, 0<y<1), and in that the switch further comprises a temperature controller arranged to adjust the operating temperature of said KTa 1-x Nb x O 3 or K 1-y Li y Ta 1-x Nb x O 3 to the phase transition temperature at which said KTa 1-x Nb x O 3 or K 1-y Li y Ta 1-x Nb x O 3 changes from a tetragonal crystal to a cubic crystal.