A post-wall center-feed waveguide consisting of T-junctions is proposed for reducing the slot-free area of a parallel plate slot array antenna. The width of the slot-free area is reduced from 2.6 λ₀ to 2.1 λ₀. A sidelobe level in the E-plane is expected to be suppressed lower than that of the conventional center-feed antenna using cross-junctions. The method of moments with solid-wall replacement designs initially the T-junctions and HFSS including the post surfaces modifies only the reflection cancelling post. We have designed and fabricated a 61.25GHz model antenna with uniform aperture illumination. The sidelobe level in the E-plane is suppressed to -9.5dB while that of a conventional cross-junction type is -7.8dB. Also, we suppress it to -13.8dB by introducing a -8.3dB amplitude tapered distribution in the array of the radiation slot pairs.