A center-feed structure using cross-junctions is designed for a post-wall waveguide-fed parallel plate slot array antenna. The length of the radiating waveguide in the center-feed antenna is half of that of a conventional end-feed antenna, and the center-feed antenna is expected to enhance bandwidth depending on the long line effect. The symmetrical structure of the antenna can keep the main beam at boresight as the frequency is changed. The cross-junction is analogous to that previously proposed for a single-mode waveguide array, but it excites plane TEM waves in both directions in the oversized parallel plate. The authors have designed and fabricated a slot array with uniform aperture distribution at 61.25 GHz. The bandwidth for 1-dB gain reduction is 1.7 times as wide as that of a conventional end-feed type antenna. The sidelobe level in the E-plane increases to -7.8 dB because of the slot-free area over the center-feed waveguide, which is suppressed to -11.1 dB by introducing a -8.3 dB amplitude tapered distribution in the slot design of the array.