The present invention relates to a resin composition comprising: a polymer (I) selected from the group consisting of a polymer (a1) containing a structural unit (i) having an ester linkage type betaine structure and a polymer (b1) containing a structural unit (ii) having a betaine structure that is not of an ester linkage type and a structural unit (iii) having a carboxyl group; an organic solvent having a Hansen solubility parameter at 25° C. in which a dispersion term δD is 10 to 24 MPa1/2, a polarity term δP is 5 to 19 MPa1/2, and a hydrogen bonding term δH is 3 to 17 MPa1/2, and having a boiling point higher than 100° C.; and water.