Included are a first modulator, a second modulator, a first optical amplifier that amplifies an output of the first modulator at an amplification factor based on a first bias signal, a second optical amplifier that amplifies an output of the second modulator at an amplification factor based on a second bias signal, an optical phase adjuster that phase-rotates an output of the second optical amplifier, an optical multiplexer that multiplexes an output of the first optical amplifier with an output of the optical phase adjuster, and a second bias corrector that generates a first pulse signal and a second pulse signal, which are complementary to each other, and obtains a first bias value and a second bias value based on a change of strength of an output signal of the optical multiplexer. The first and second pulse signals are superimposed on the first and second bias signals, respectively.