We propose a millimeter-wave antenna to serve as an element antenna for a multi-sector switched-beam antenna, which can be fed by a post-wall waveguide. Several tapered slot antennas are connected to an aperture of an H-plane sectoral post-wall horn antenna whose radiation pattern is narrowed in the H-plane to further narrow the pattern in the E-plane. Moreover, the aperture of each tapered slot antenna is loaded with a dielectric to increase the gain. We designed an antenna whose gain is over 16 dBi in the frequency band of 57-66 GHz. The wideband property is experimentally examined.