A hydrophilic surface treatment agent for an aluminum-containing metallic heat exchanger, which is obtained by mixing: a water-soluble resin (A) having at least one or more functional groups of an amide group, a hydroxyl group, and a carboxyl group, or a water-soluble resin (A) including, in a skeleton thereof, an amide linkage; colloidal silica (B); organo alkoxy silane and/or a hydrolysate thereof (C); a cross-linking agent (D) capable of forming cross-linkage with the water-soluble resin (A); and water (E), wherein the ratio {(B)+(C)}/{(A)+(B)+(C)+(D)} is 0.1 to 0.5 in terms of solid content ratio (mass ratio), and the ratio (C)/(B) is 0.5 to 4.0 in terms of solid content ratio (mass ratio).