PURPOSE: To provide a multi-mode slab waveguide having high end fire connection efficiency of light from a fiber, a laser diode or an objective lens for a microsocope.

CONSTITUTION: In an waveguide structure 20 including a substrate 22, an waveguide film 24 and an overlay material 26 brought into contact with the film 24, the material 26 is brought into contact with the substrate 22 or the film 24 is arranged between the material 26 and the substrate 22. The material 26 has a refractive index higher than that of the waveguide 24 so that the ratio of effective refractive indexes in all propagation modes supported by the waveguide 24 is substantially fixed.