This is to provide a coating film having a function of inhibiting adhesion of a biological substance, a method for manufacturing the coating film, a copolymer obtainable by polymerizing a specific monomer mixture, a composition for forming a coating film having a specific composition, a method for manufacturing a varnish containing a copolymer to be used as a raw material of the composition for forming a coating film which is used for forming said film, and a sol for forming the coating film. In particular, this is to provide a coating film obtained by the method comprising a process of applying a composition for forming a coating film which comprises a copolymer comprising a recurring unit containing an organic group of the formula (a) and a recurring unit containing an organic group of the formula (b) and a solvent onto a substrate; and a process of drying at a temperature of −200° C. to 200° C. (wherein Ua1, Ua2, Ub1, Ub2 and Ub3, and An− are as defined in the present specification and the claims).