Biodegradable polymer is key material for realizing a sustainable society. It is also useful for making minimally invasive surgical devices that resolve in the body. Although micro mechanical structures of biodegradable polymer are needed, few micromachining processes were reported. We proposed and developed precise etching method of Poly (L-Lactide), one of the biodegradable polymers, by using oxygen reactive ion etching and flexible stencil mask that was made of polyimide film. This etching method does not damage the polymer because of low temperature process and the use of mild organic solvents. Etching characteristics and process condition were investigated. Maximum and minimum etching errors were 15μm and 1μm, respectively.