An optical sender is disclosed that operates in a Differential Quadrature Phase Shift Keying modulation scheme for high speed optical transmission and is capable of performing logical calculations at a low speed. The optical sender transmits a Differential Quadrature Phase Shift Keying (DQPSK) signal generated with modulation signals ρk and ηk so that a signal directly output from a signal receiver corresponding to the optical sender is in agreement with data signals Ik and Qk to be transmitted. The signal receiver is capable of modulation by DQPSK, and the modulation signals ρk and ηk are precoded by using the data signals Ik and Qk and the modulation signals one symbol earlier (ρk−1 and ηk−1) . The optical sender includes plural precoders that perform logical calculation simultaneously and in parallel on plural data signals one period after another period.