The optical transmission apparatus has a Mach-Zehnder optical modulator, a light source for applying an optical signal of a continuous light to the optical modulator, a driving circuit for inputting a driving signal into the optical modulator, a branching filter for taking out a part of an output optical signal, a photodiode for converting the taken-out output optical signal into an electric signal, a band pass filter for extracting a frequency component of a driving signal included in this electric signal, an error signal generator circuit for generating an error signal of a bias voltage that minimizes a value of the extracted frequency component, and a bias voltage control circuit for applying a bias voltage added with this error signal to the optical modulator.