An optical transmitter configured to generate a polarization-multiplexed optical signal characterized by comprising: a band division unit configured to divide a band of a transmission data sequence, said transmission data sequence is divided into two data sequences, each data sequence corresponding to the data to be modulated into an x-polarization signal and in a y-polarization signal, and each data sequence comprising an in-phase component and a quadrature component, the band division unit is configured to output for each data sequence a first signal comprising an in-phase and a quadrature component corresponding to a band included in an upper sideband among bands obtained by the division and a second signal comprising an in-phase component and a quadrature component corresponding to a band included in a lower sideband among the bands obtained by the division; a digital-to-analog converter configured to convert the first and second signals for each data sequence from digital signals to analog signals to generate third and fourth signals; a first narrowband signal processing unit configured to input the third signal and the fourth signal for each data sequence and output first, second, third and fourth narrowband signals, the first narrowband signal representing a sum of in-phase components of the third and fourth signals, the second narrowband signal representing a difference between quadrature components of the third and fourth signals, the third narrowband signal representing a difference between the in-phase components of the third and fourth signals, and the fourth narrowband signal representing a sum of the quadrature components of the third and fourth signals; a wideband signal generation unit configured to multiply the first and second narrowband signals for each data sequence by sinusoidal signals having a phase difference of (π/2), respectively, to shift bands of the first and second narrowband signals, and combine the shifted first and second narrowband signals to generate a first wideband signal, the wideband signal generation unit being configured to multiply the third and fourth narrowband signals for each data sequence by sinusoidal signals having a phase difference of (π/2), respectively, to shift bands of the third and fourth narrowband signals, and combine the shifted third and fourth narrowband signals to generate a second wideband signal; and an optical modulator configured to modulate an optical signal using the first and second wideband signals for each data sequence and output the modulated polarization-multiplexed optical signal. a band division unit configured to divide a band of a transmission data sequence, said transmission data sequence is divided into two data sequences, each data sequence corresponding to the data to be modulated into an x-polarization signal and in a y-polarization signal, and each data sequence comprising an in-phase component and a quadrature component, the band division unit is configured to output for each data sequence a first signal comprising an in-phase and a quadrature component corresponding to a band included in an upper sideband among bands obtained by the division and a second signal comprising an in-phase component and a quadrature component corresponding to a band included in a lower sideband among the bands obtained by the division; a digital-to-analog converter configured to convert the first and second signals for each data sequence from digital signals to analog signals to generate third and fourth signals; a first narrowband signal processing unit configured to input the third signal and the fourth signal for each data sequence and output first, second, third and fourth narrowband signals, the first narrowband signal representing a sum of in-phase components of the third and fourth signals, the second narrowband signal representing a difference between quadrature components of the third and fourth signals, the third narrowband signal representing a difference between the in-phase components of the third and fourth signals, and the fourth narrowband signal representing a sum of the quadrature components of the third and fourth signals; a wideband signal generation unit configured to multiply the first and second narrowband signals for each data sequence by sinusoidal signals having a phase difference of (π/2), respectively, to shift bands of the first and second narrowband signals, and combine the shifted first and second narrowband signals to generate a first wideband signal, the wideband signal generation unit being configured to multiply the third and fourth narrowband signals for each data sequence by sinusoidal signals having a phase difference of (π/2), respectively, to shift bands of the third and fourth narrowband signals, and combine the shifted third and fourth narrowband signals to generate a second wideband signal; and an optical modulator configured to modulate an optical signal using the first and second wideband signals for each data sequence and output the modulated polarization-multiplexed optical signal.