We have developed a new wafer process for manufacturing micro-structures using Micro Electrical Mechanical Systems (MEMS) technology. Our process combines a single-crystal SOI wafer, vertical Si etching, and back side anisotropic-etching by KOH, and dry release by the plasma etching of SiO2. The sticking of movable structures cannot occur in our process, because no wet treatment is used for release. Our process needs only 4-masks, therefore, it is suitable for mass-production. We have developed ultra-slim acceleration sensor using this process.