To provide a positive photoresist composition having improved marginal resolving power, development defects and linearity.

The positive photoresist composition contains a polymer obtained by reacting a polymer having phenolic hydroxyl groups with an alkyl vinyl ether compound and an alcohol compound in the presence of an acidic catalyst and further reacting the resulting reaction product with an alkyl vinyl ether compound whose structure is different from that of the above alkyl vinyl ether compound and having solubility in an alkali developing solution increased by decomposition by the action of an acid, a compound which generates the acid when irradiated with active light or radiation and a solvent.