A planar antenna comprised of parallel feed waveguide circuit and sub-arrays of four aperture elements have been proposed. First, we describe a fundamental concept of planar antenna employing parallel feed waveguide. Next, a parallel feed circuit using rectangular waveguide, a subarray with four aperture elements, new T-junction which is made by casting in one body and L-bend is designed, and characteristics of the planar antenna is investigated experimentally. The antenna, proposed here, has the feature of a good aperture efficiency, broadband, high heat-resistance and small size. The aperture efficiency is over 90% in frequency ratio bandwidth of 5.8% and is over 80% in frequency ratio bandwidth of 11.0%.