PURPOSE: To superpose a monitoring and controlling low frequency signal on a main signal with a relatively simple constitution without using plural optical modulators.

CONSTITUTION: This device is provided with a light source 31 sending coherent light, an external optical modulator 33 passing the light sent from the light source 31 through an optical waveguide 331 and modulating the light passing through the optical waveguide 331 by an electrooptic effect by allowing an electric signal to flow to an electrode 332 for modulation arranged on the optical waveguide 331, a differential amplifier differentially amplifying the electric signal to the external optical modulator 33 and a drive circuit 35 connecting the output end of the differential amplifier to one side end of the electrode 332 for modulation and terminating the other side end, and by variably controlling the drive current of the differential amplifier with the monitoring and controlling low frequency signal, the device superposes the monitoring and controlling signal on the electric signal used as the main signal.