In this letter, a novel antenna with a miniaturized aperture is proposed. The substrate including a split-ring resonator array is inserted into a size-reduced open-ended waveguide. For a low return loss and high radiation efficiency, the ring arrangement is optimized, and a stepped transition using H-plane discontinuity is proposed. The proposed antenna achieves a 70% aperture reduction compared to a conventional standard waveguide antenna of WR-187 (47.6 mm${ imes}$22.2 mm). The return loss drops significantly at three frequencies, and a reasonable gain is achieved. The aperture-miniaturized antenna can be used in many antenna applications such as near-field measurement.