A process of forming a deflection mirror in a light waveguide with a use of a dicing blade having a cutting end with a flat top cutting face and at least one slanted side cutting face. The process includes a cutting step of cutting a surface of the light waveguide to a depth not greater than a width of the flat top cutting face, thereby forming a groove in the surface of the light waveguide. The groove has a slanted surface which is formed by the slanted cutting face to define the deflection mirror in the waveguide.