An optical waveguide device formed by a material showing anisotropy for refraction index and an optical instrument using the same device. The device comprises an electro-optic single crystal substrate on which are formed a first core portion higher in refractive index than the substrate with respect to both ordinary and extraordinary rays and a second core portion higher in refractive index than the substrate with respect to extraordinary rays pnly whereby the first core portion forms a single-moded waveguide for ordinary rays, and the overlapping region of the first and second core portions forms a double-moded waveguide for extraordinary rays. This optical waveguide device is used in place of a pinhole elementin an optical system of a confocal scanning optical microscopeso as to guide illuminating light to an object to be detected and to guide the reflected light from the object into a detector. This microscope includes a polarization conversion element for converting the polarizing direction of the illuminating light to one different from that of reflected light. The optical waveguide device functions as a single-moded waveguide for the illuminating light and as a double-moded waveguide for the reflected light.