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Millimeter-wave (MMW) active imaging has potentially a variety of applications. However, MMW circuits are often lossy and amplifiers are costly, which is a serious problem in particular in arrays using many front-end circuits in parallel. To overcome this point, there have been some proposals such as the envelope phase detection (EPD) method where MMW is directly detected without amplification just after received at the antenna. To realize a high sensitivity, this communication proposes a MMW imaging front-end using a bulk tapered slot antenna (bulk TSA). Since bulk TSAs possess an intrinsically low impedance characteristic, we can obtain a high detection power. Experiments demonstrate that a front-end using 1 mm-thick TSA achieves 22 dB-higher sensitivity than a conventional circuit using 0.06 μm TSA. We analyze the front-end numerically to elucidate its characteristics. We also investigate array configurations to find that a coplanar arrangement exhibits higher isolation than a stacked configuration.