PURPOSE: To obtain a miniature frequency converting circuit having high performance and low power consumption, by modulating the phase of light wave by a micorwave signal and a local oscillation signal.

CONSTITUTION: The laser beam oscillated by exciting a semiconductor laser 3 with a DC is led to a waveguide 2 through an electrooptical crystal edge and branched with the equal intensity through a Y branching filter 11 to go to optical waveguides 12 and 13 which form a phase modulator. The beams going to the waveguides 12 and 13 receive the modulation of phase by a microwave signal 7 and a local oscillation signal 8 respectively. These two light waves are synthesized through a 3dB coupler 14 and then receive the heterodyne wave detection through a photodetector. The signal 10 detected by the heterodyne wave passes through an electrical filter that trnasmits only the differential frequency between the signals 7 and 8. Thus the microwave signal is exchanged to an intermediate frequency band. In such a way, a wide band width and a miniature structure are attained with low power consumption for a frequency converting circuit.