To provide a positive photosensitive composition for exposure with far UV adaptable to exposure in the far UV region using ArF or KrF as a light source in production of a semiconductor device, ensuring improved dimensional uniformity of a resist pattern and excellent also in various properties as a resist, such as resolving power, focal depth and development defects.

The positive resist composition is characterized in that it contains (a) a polymer which has a silicon atom in a side chain, is insoluble or slightly soluble in an aqueous alkali solution and is made soluble in the aqueous alkali solution by the action of an acid and (b) a compound which generates an acid upon irradiation with an actinic ray or a radiation and has a specified phenacylsulfonium structure.