A rheology control agent for water-based resins may include: 1 mass % or more and 30 mass % or less of an active matter (X) containing a diamide compound (A) and/or a hydrogenated castor oil (A′), the diamide compound (A) being obtained by reacting a diamine component (A1) selected from the group consisting of diamines having 2 to 12 carbon atoms with a monocarboxylic acid component (A2) selected from the group consisting of monocarboxylic acids having at least a hydroxyl group; and 5 mass % or more and 99 mass % or less of a first solvent (C) having at least one substituent selected from the group consisting of a hydroxyl group, an ether group, an ester group, an amide group and a ketone group.