An optical modulator, comprising: a first optical waveguide and a second optical waveguide formed of InP, materials capable of lattice matching with InP, or materials of which the degree of lattice mismatching with InP is within 1.0%, and formed on an InP substrate and constituting an optical interference unit; a first slot line electrode formed at a side of said first optical waveguide, the first slot line electrode being connected to receive a high-frequency signal from a high-frequency driving power supply; a second slot line electrode formed at a side of said second optical waveguide and having wider width than the first slot line electrode, the second slot line electrode being connected to a ground terminal of the high-frequency driving power supply; a plurality of first discrete electrodes discretely formed on said first optical waveguide and connected to said first slot line electrode; and a plurality of second discrete electrodes discretely formed on said second optical waveguide and connected to said second slot line electrode, wherein a plurality of first phase modulators are constituted by the plurality of first discrete electrodes and the first optical waveguide located below the plurality of first discrete electrodes, and wherein a plurality of second phase modulators are constituted by the second discrete electrodes and the second optical waveguide located below the second discrete electrodes, characterised in that a total operation length obtained by adding up operation lengths of the plurality of second phase modulators is constituted to be longer than a total operation length obtained by adding up operation lengths of the plurality of first phase modulators, and a total number of said second discrete electrodes is constituted to be larger than a total number of said first discrete electrodes, wherein, with a total electrode length obtained by adding up respective lengths of said plurality of second discrete electrodes set as LG, a total electrode length obtained by adding up respective lengths of said plurality of first discrete electrodes set as LS, and an electrode length asymmetry parameter set as r = (LG - LS) / (LG + LS), said plurality of first discrete electrodes and said plurality of second discrete electrodes are formed to make a value of the electrode length asymmetry parameter r within a range of 0.1 to 0.3. a first optical waveguide and a second optical waveguide formed of InP, materials capable of lattice matching with InP, or materials of which the degree of lattice mismatching with InP is within 1.0%, and formed on an InP substrate and constituting an optical interference unit; a first slot line electrode formed at a side of said first optical waveguide, the first slot line electrode being connected to receive a high-frequency signal from a high-frequency driving power supply; a second slot line electrode formed at a side of said second optical waveguide and having wider width than the first slot line electrode, the second slot line electrode being connected to a ground terminal of the high-frequency driving power supply; a plurality of first discrete electrodes discretely formed on said first optical waveguide and connected to said first slot line electrode; and a plurality of second discrete electrodes discretely formed on said second optical waveguide and connected to said second slot line electrode, wherein a plurality of first phase modulators are constituted by the plurality of first discrete electrodes and the first optical waveguide located below the plurality of first discrete electrodes, and wherein a plurality of second phase modulators are constituted by the second discrete electrodes and the second optical waveguide located below the second discrete electrodes, characterised in that a total operation length obtained by adding up operation lengths of the plurality of second phase modulators is constituted to be longer than a total operation length obtained by adding up operation lengths of the plurality of first phase modulators, and a total number of said second discrete electrodes is constituted to be larger than a total number of said first discrete electrodes, wherein, with a total electrode length obtained by adding up respective lengths of said plurality of second discrete electrodes set as LG, a total electrode length obtained by adding up respective lengths of said plurality of first discrete electrodes set as LS, and an electrode length asymmetry parameter set as r = (LG - LS) / (LG + LS), said plurality of first discrete electrodes and said plurality of second discrete electrodes are formed to make a value of the electrode length asymmetry parameter r within a range of 0.1 to 0.3.