There is disclosed a chemically amplified positive resist composition to form a chemically amplified resist film to be used in a lithography, wherein the chemically amplified positive resist composition comprises at least, (A) a base resin, insoluble or poorly soluble in an alkaline solution, having a repeating unit whose phenolic ydroxyl group is protected by a tertiary alkyl group, while soluble in an alkaline solution when the tertiary alkyl group is removed; (B) an acid generator; (C) a basic component; and (D) an organic solvent, and a solid component concentration is controlled so that the chemically amplified resist film having the film thickness of 10 to 100 nm is obtained by a spin coating method. There can be provided, in a lithography, a chemically amplified positive resist composition giving a high resolution with a suppressed LER deterioration caused by film-thinning at the time of forming a chemically amplified resist film with the film thickness of 10 to 100 nm, and a resist patterning process using the same.