An optical transmitter includes quadrature modulators and light receiving elements to which inverted output light of output light from the quadrature modulators is input, the quadrature modulators including parent Mach-Zehnder modulators in respective paths of a first pair of paths into which carrier light from a light source is split, the parent Mach-Zehnder modulators including child Mach-Zehnder modulators including first phase modulation units, and second phase modulation units. When blocking a transmission optical signal, the voltage amplitude of a transmission electrical signal to be applied to the quadrature modulator is adjusted such that it is smaller than a half-wave voltage, at most two dither signals are applied to the first phase modulation units, a component output by the light receiving element, the component having n times a frequency of the dither signals, is detected, and bias voltages to be applied to the first phase modulation units are controlled such that the component having n times the frequency is minimized.