Reaction development patterning (RDP) was applied to poly(lactic acid) (PLA) to realize formation of a fine pattern by a photolithography process. The RDP of PLA using an aqueous solution of tetramethylammonium hydroxide (TMAH) as a developer gave a positive-tone fine pattern. It was also revealed that addition of polyfunctional aromatic acids such as 3,5-dihydroxybenzoic acid to the film of PLA largely shortened the development time. When an alcoholic solution of TMAH was used as a developer, a negative-tone fine pattern was obtained, and pattern formation was achieved by a very short-time development without adding an acidic compound to the film.