PURPOSE: To equalize the intensity of the light signal from a waveguide, in the signal processing circuit of a waveguide device, by providing a gain control means compensating the non-equality of the propagation loss at every waveguide.

CONSTITUTION: The beam from a laser diode 1 is reflected by a half mirror 2 to be condensed to the surface of an object 5 by an object lens 4 through an XY two-dimensional scanning means 3. The reflected beam transmits through the mirror 2 to be condensed to a double mode waveguide 7. This beam is distributed to two waveguides 9, 10 and the distributed beams are respectively incident on photodetectors 11, 12 and electric signals having the intensity proportional to beam power is sent to amplifiers 13, 14. A differential amplifier circuit 15 detects the difference between both amplified outputs to output the signal corresponding to said difference. The output signals of the photodetectors 11, 12 are different in detection intensity because the propagation loss factors of the waveguides 9, 10 are different. The gain rates of the amplifiers 13, 14 are successively altered to be determined so that the output from the differential amplifier circuit 15 becomes zero. As mentioned above, by setting the gain rates and separately amplifying the same by the amplifiers 13, 14, the light powers emitted from a plurality of waveguides are equalized to be accrately detected.