To provide a method for preparing a resin mixture liquid with fewer agglomerates.

The method for preparing a resin mixture liquid comprises the following steps (1)-(3) in order. Step (1): preparing a first mixture liquid containing a hydroxy group-containing resin (A), an ion exchange resin and a solvent; step (2): preparing a second mixture liquid by mixing a polycarboxylic acid neutralized material (B) to the first mixture liquid; and step (3): removing the residue of the ion exchange resin from the second mixture liquid. Further, in the step (1), the first mixture liquid further includes an inorganic layered compound in addition to the hydroxy group-containing resin (A), the ion exchange resin and the solvent.