To prepare a thin film-forming functional polymer usable for hotoresist, or the like and having dicarboxylic acid half ester groups by including a specific hydrolysis resistant polymer ingredient and a specific heat-resistant polymer ingredient.

This thin film-forming functional polymer having 1,2-dicarboxylic acid half ester groups is prepared by including (A) 1-99 mol.% (pref. 5-50 mol.%) polymer ingredient having acid lability and hydrolytic resistance, selected from compounds of formula I, II [(n) is 0-3; R is 2-tetrahydrofuranyl, 2- tetrahydropyranyl, or the like], or the like and (B) 99-1 mol.% heat-resistant polymer ingredient of formula III and IV [R2 to R4 are each a 1-6C alkyl, a 1-6C alkoxy, or the like; R5 is H or a 1-6C alkyl; X is Si or Sn; Y is O or NH] and, optionally, (C) 1-50 mol.% heat-resistant polymer ingredient of formula V (R6 to R9 are each H, a 1-6C alkyl, a halogen, or the like).