In photonics, light usually diffracts in all directions when it emerges from a planar optical waveguide. Besides this fact, in this letter we show that a waveguide with a rectangular cross section can be turned to a focusing system by using three-dimensional self-imaging technique. We obtained a configuration where the focus of the field lies outside the waveguide, in air, with a spot size of approximately the resolution limit of half a wavelength. This type of waveguide could be used as a coupling element in integrated optics or in high numerical aperture optical systems.