Electromagnetic properties of a planar metallic metasurface with the design inspired by Babinet's principle are numerically studied. The metasurface is constructed from a metal plate perforated by coaxial-sector apertures. It is shown that the chosen coaxial-sector apertures make it possible to obtain a wider operating range of the metasurface than those composed of apertures of other shapes (e.g. round or rectangle). Moreover, the proposed metasurface performs an efficient polarization conversion of the linearly polarized wave to elliptically and circularly polarized ones in the reflected field.