An optical transmitting apparatus includes a modulating unit that branches an input light and performs independent phase modulation to branched optical signals of arms, a phase adjusting unit that changes a phase difference between the optical signals of respective arms according to a control signal, a combining unit that combines modulated lights having the phase difference, an acquiring unit that acquires a positive-phase signal and a negative-phase signal from the combining unit, a subtracting unit that obtains a difference between the positive-phase signal and the negative-phase signal acquired by the acquiring unit, a detecting unit detecting a power of a differential signal from subtraction by the subtracting unit, and a control unit that changes the control signal according to signal component intensity detected by the detecting unit.