A polymer suitable as a base resin for a positive resist composition, in particular a chemically amplified positive resist composition, having a higher resolution, a larger exposure allowance, a smaller sparse-dense size difference, a better process applicability, a better pattern configuration after exposure, and in addition, a further excellent etching resistance, than a conventional positive resist. A positive resist composition using the same, a patterning process, and a novel polymerizable compound to obtain the polymer. A polymer has a hydrogen atom of at least a carboxyl group is substituted by an acid labile group represented by the following general formula (2), a positive resist composition using the same, a patterning process, and a novel polymerizable compound to obtain a polymer like this.