In a chemically amplified positive resist composition comprising a base resin and an acid generator in a solvent, the base resin contains both an alkali-insoluble or substantially alkali-insoluble polymer having an acid labile group-protected acidic functional group having a Mw of 1,000-500,000 and an alkyl vinyl ether polymer having a Mw of 10,000-500,000. The composition forms on a substrate a resist film of 5-100 μm thick which can be briefly developed to form a pattern at a high sensitivity and a high degree of removal or dissolution to bottom.