To realize a high-density spin-transfer-torque magnetic random access memory (STT-MRAM) device comparable with a current dynamic random access memory (DRAM) device, it is a key to develop a new technology for memory cell size reduction. We have already reported a chemical- mechanical-polishing(CMP)-based preparation technology for magnetic tunnel junctions (MTJs) above the via holes that can drastically reduce memory cell area. In this paper, we first introduce the MTJ preparation technology to the mega-bit class STT-MRAM test chip, and demonstrate the improvement of memory-cell operation yield.