In an optical synthesizer, a laser source outputs a laser. An optical modulator modulates the frequency of the laser to output a light including a first frequency component. An optical filter extracts the first frequency component from the output of the optical modulator. An optical comb generator generates an optical comb based on the laser and a predetermined driving signal. A variable-wavelength narrowband filter extracts a second frequency component from the optical comb. An optical-electric converter outputs an electric signal based on the frequency difference between the first and second frequency components.