To provide a photoresist resin which can form a fine pattern with excellent homogeneity and high resolution.

The polymer to be used for the photoresist resin contains at least one kind of monomer unit expressed by formula (I). In the formula, Ra is a hydrogen atom or methyl group, Rb represents one or more substituents which may be same or different and each Rb is a hydrogen atom, methyl group, hydroxyl group which may have protecting groups, carboxyl group which may have protecting groups or oxo group and R1 is a 1-8C straight chain, branched or cyclic hydrocarbon group.