Abstract: The invention provides a composition for forming a coating film which comprises (1) a polymer of a monomer mixture containing monomers represented by the following formulae (A) to (D): (wherein Ta, Tb, Tc, Td, Ua1, Ua2, Ub1, Ub2, Ub3, Qa, Qb, Qc, Ra, Rb, Rc, Rd, An?, m and n are as defined herein), where a ratio of a total of the anionic monomer represented by the above-mentioned formula (A) and the cationic monomer represented by the above-mentioned formula (B) based on total monomers contained in the monomer mixture is 40 mol % or more, and (2) a polycarbodiimide containing a structure represented by the following formula (E): —N?C?N—??(E) a coating film which is a cured product thereof and a process for producing the same, and a cured product and a process for producing the same.