In the present invention, a coating film is formed by a coating-film forming composition being contacted with a base and subsequently being dried. The coating-film forming composition contains a solvent and a copolymer that includes a repeat unit containing an organic group expressed by formula (a) and a repeat unit containing an organic group expressed by formula (b) (in the formula, Ua1 represents a hydrogen atom or an alkali metal atom, and Ub1 and Ub2 each independently represent a hydrogen atom, or a linear or branched alkyl group having 1-5 carbon atoms).一種塗膜，其係使包含共聚物及溶媒之塗膜形成用組成物接觸至基體後，予以乾燥而形成，該共聚物包含：含下述(式a)所示之有機基的重複單元、及含下述(式b)所示之有機基的重複單元；(式中，Ua1表示氫原子或鹼金屬原子，Ub1及Ub2各自獨立地表示氫原子或碳原子數1~5之直鏈或分枝烷基)。
