An optical transmitter (10, 10A, 10B, 10C, 10D)comprising an optical phase modulator, a phase shifter, a photodetector, and a controller, characterized in the optical transmitter further comprising: a) the optical phase modulator including a first waveguide and a second waveguide and configured to modulate carrier light with a modulation driving signal having 2*n intensity levels (where n is an integer 1 or greater) at each of the first and second waveguides (181, 18Q); b) the phase shifter configured to cause a predetermined phase difference between a first optical signal output from the first waveguide and a second optical signal output from the second waveguide; c) the photodetector configured to detect a portion of a multilevel optical modulation signal acquired by combining the first optical signal and the second optical signal having the phase difference and perform optical-to-electrical conversion on the detected modulation signal; d) a monitor (39, 49, 59) configured to detect a change in an alternating current component in the detected modulation signal; e) the controller (25, 35, 45, 55, 65) configured to control at least one of a first bias voltage being supplied to the first waveguide and a second bias voltage being supplied to the second waveguide so as to increase a power value of the alternating current component based upon the change in the alternating current component; and f) wherein the controller (25, 35, 45, 55, 65) further controls a third bias voltage being supplied to the phase shifter so as to decrease an electric power spectrum of the alternating current component, and g) wherein the controller (25, 35, 45, 65) controls the first bias voltage, the second bias voltage, and the third bias voltage simultaneously or in a time-shared manner. a) the optical phase modulator including a first waveguide and a second waveguide and configured to modulate carrier light with a modulation driving signal having 2*n intensity levels (where n is an integer 1 or greater) at each of the first and second waveguides (181, 18Q); b) the phase shifter configured to cause a predetermined phase difference between a first optical signal output from the first waveguide and a second optical signal output from the second waveguide; c) the photodetector configured to detect a portion of a multilevel optical modulation signal acquired by combining the first optical signal and the second optical signal having the phase difference and perform optical-to-electrical conversion on the detected modulation signal; d) a monitor (39, 49, 59) configured to detect a change in an alternating current component in the detected modulation signal; e) the controller (25, 35, 45, 55, 65) configured to control at least one of a first bias voltage being supplied to the first waveguide and a second bias voltage being supplied to the second waveguide so as to increase a power value of the alternating current component based upon the change in the alternating current component; and f) wherein the controller (25, 35, 45, 55, 65) further controls a third bias voltage being supplied to the phase shifter so as to decrease an electric power spectrum of the alternating current component, and g) wherein the controller (25, 35, 45, 65) controls the first bias voltage, the second bias voltage, and the third bias voltage simultaneously or in a time-shared manner.