To provide a polymer for photoresist suitable for short wavelength lithography, excellent in performance such as resolution and sensitivity and having a satisfactory line width roughness, forming a lithograph with satisfactory productivity and operability, and to provide a photoresist composition comprising the same.

The polymer comprises a structural unit represented by formula (I), and a structural unit represented by formula (II), wherein X represents a linear or branched 1-6C alkylene. The photoresist composition comprises the polymer and an acid generator generating an acid by light exposure.