An optical signal transmitter includes first and second modulation units, a combiner, and a control unit. The first and second modulation units generate first and second modulated optical signals, respectively. The combiner combines the first and second modulated optical signals to generate a polarization multiplexed optical signal. The control unit controls at least one of the first and second modulation units so that the optical powers of the first and second modulated optical signals become approximately equal to each other.