An optical transmission system including a transmitter, a receiver, and an optical fiber transmission line, the transmitter and the receiver being connected to each other via the optical fiber transmission line, signal light being transmitted through the optical fiber transmission line, wherein the transmitter is configured to generate signal light in which an optical carrier signal of an optical frequency f1 is modulated and multiplexed with orthogonal polarization using two data signal sequences and send out the signal light to the optical fiber transmission line, and the receiver is configured to have two coherent receivers that perform coherent detection of the signal light using two pieces of local oscillator of optical frequencies f11 and f12 close to the optical frequency f1 of the signal light, f11 < f12 holding, the two pieces of local oscillator being controlled to have a predetermined optical frequency spacing ΔF, and a digital signal processor that performs digital signal processing by inputting electric signals output from each of the coherent receivers and demodulates the two data signal sequences, in which the digital signal processor sets a virtual reference frequency f1' close to the optical frequency f1 of the signal light for the two pieces of local oscillator and a frequency difference Δf1 of one of the two pieces of local oscillator from the reference frequency f1', obtains a frequency difference Δf2 of the other of the two pieces of local oscillator by calculating Δf1 - ΔF, and further includes a plurality of phase rotation compensation circuits that input the electric signals output from the two coherent receivers and compensates phase rotation caused in the electric signals by the frequency differences Δf1 and Δf2, a waveform equalization circuit that performs adaptive equalization on outputs from the plurality of phase rotation compensation circuits, and a phase estimation circuit that compensates a residual component of phase rotation caused by a frequency difference between the optical frequency f1 of the signal light and the virtual reference frequency f1' in an output from the waveform equalization circuit. the transmitter is configured to generate signal light in which an optical carrier signal of an optical frequency f1 is modulated and multiplexed with orthogonal polarization using two data signal sequences and send out the signal light to the optical fiber transmission line, and the receiver is configured to have two coherent receivers that perform coherent detection of the signal light using two pieces of local oscillator of optical frequencies f11 and f12 close to the optical frequency f1 of the signal light, f11 < f12 holding, the two pieces of local oscillator being controlled to have a predetermined optical frequency spacing ΔF, and a digital signal processor that performs digital signal processing by inputting electric signals output from each of the coherent receivers and demodulates the two data signal sequences, in which the digital signal processor sets a virtual reference frequency f1' close to the optical frequency f1 of the signal light for the two pieces of local oscillator and a frequency difference Δf1 of one of the two pieces of local oscillator from the reference frequency f1', obtains a frequency difference Δf2 of the other of the two pieces of local oscillator by calculating Δf1 - ΔF, and further includes a plurality of phase rotation compensation circuits that input the electric signals output from the two coherent receivers and compensates phase rotation caused in the electric signals by the frequency differences Δf1 and Δf2, a waveform equalization circuit that performs adaptive equalization on outputs from the plurality of phase rotation compensation circuits, and a phase estimation circuit that compensates a residual component of phase rotation caused by a frequency difference between the optical frequency f1 of the signal light and the virtual reference frequency f1' in an output from the waveform equalization circuit. sets a virtual reference frequency f1' close to the optical frequency f1 of the signal light for the two pieces of local oscillator and a frequency difference Δf1 of one of the two pieces of local oscillator from the reference frequency f1', obtains a frequency difference Δf2 of the other of the two pieces of local oscillator by calculating Δf1 - ΔF, and further includes a plurality of phase rotation compensation circuits that input the electric signals output from the two coherent receivers and compensates phase rotation caused in the electric signals by the frequency differences Δf1 and Δf2, a waveform equalization circuit that performs adaptive equalization on outputs from the plurality of phase rotation compensation circuits, and a phase estimation circuit that compensates a residual component of phase rotation caused by a frequency difference between the optical frequency f1 of the signal light and the virtual reference frequency f1' in an output from the waveform equalization circuit.