A surface treatment agent for a zinc or zinc alloy coated steel sheet, comprising: resin compound having a bisphenol skeleton represented by general formula below; cationic urethane resin emulsion having at least one type of cationic functional group selected from primary amine, secondary amine, tertiary amine and quaternary ammonium salt; at least one type of silane coupling agent having at least one type of reactive functional group selected from active hydrogen-containing amino group, epoxy group, mercapto group and methacryloxy group; organic titanium chelate compound; quadrivalent vanadyl compound; molybdic acid compound; and water, such that conditions to below are satisfied, pH of the surface treatment agent being in the range of 4 to 5, wherein: [(B s)/{(A s) + (B s) + (C s)}] as a mass ratio of solid content (B s) of the cationic urethane resin emulsion with respect to total solid content {(A s) + (B s) + (C s)} of the resin compound, the cationic urethane resin emulsion and the silane coupling agent is in the range of 0.1 to 0.3; [(C s)/{(A s) + (B s) + (C s)}] as a mass ratio of solid content (C s) of the silane coupling agent with respect to total solid content {(A s) + (B s) + (C s)} of the resin compound, the cationic urethane resin emulsion and the silane coupling agent is in the range of 0.6 to 0.85; {(C s)/(D Ti)} as a mass ratio of solid content (C s) of the silane coupling agent with respect to mass content (D Ti) in terms of titanium of the organic titanium chelate compound is in the range of 50 to 70; {(E V)/(D Ti)} as a mass ratio of mass content (E V) in terms of vanadium of the quadrivalent vanadyl compound with respect to mass content (D Ti) in terms of titanium of the organic titanium chelate compound is in the range of 0.3 to 0.5; [(F Mo)/{(A s) + (B s) + (C s)}] as a mass ratio of mass content (F Mo) in terms of molybdenum of the molybdic acid compound with respect to total solid content {(A s) + (B s) + (C s)} of the resin compound, the cationic urethane resin emulsion and the silane coupling agent is in the range of 0.003 to 0.03: in formula, Y 1 and Y 2 bonded to benzene rings, respectively, are each independently hydrogen or group z represented by general formula or below, the average substitution number of the group z per benzene ring is in the range of 0.2 to 1.0, and n represents integer in the range of 2 to 50: in formula and formula, R 1, R 2, R 3, R 4 and R 5 each independently represent hydrogen atom, C 1-10 alkyl group or C 1-10 hydroxyalkyl group, and A - represents hydroxyl ion or acid ion. resin compound having a bisphenol skeleton represented by general formula below; cationic urethane resin emulsion having at least one type of cationic functional group selected from primary amine, secondary amine, tertiary amine and quaternary ammonium salt; at least one type of silane coupling agent having at least one type of reactive functional group selected from active hydrogen-containing amino group, epoxy group, mercapto group and methacryloxy group; organic titanium chelate compound; quadrivalent vanadyl compound; molybdic acid compound; and water, [(C s)/{(A s) + (B s) + (C s)}] as a mass ratio of solid content (C s) of the silane coupling agent with respect to total solid content {(A s) + (B s) + (C s)} of the resin compound, the cationic urethane resin emulsion and the silane coupling agent is in the range of 0.6 to 0.85; {(C s)/(D Ti)} as a mass ratio of solid content (C s) of the silane coupling agent with respect to mass content (D Ti) in terms of titanium of the organic titanium chelate compound is in the range of 50 to 70; {(E V)/(D Ti)} as a mass ratio of mass content (E V) in terms of vanadium of the quadrivalent vanadyl compound with respect to mass content (D Ti) in terms of titanium of the organic titanium chelate compound is in the range of 0.3 to 0.5; [(F Mo)/{(A s) + (B s) + (C s)}] as a mass ratio of mass content (F Mo) in terms of molybdenum of the molybdic acid compound with respect to total solid content {(A s) + (B s) + (C s)} of the resin compound, the cationic urethane resin emulsion and the silane coupling agent is in the range of 0.003 to 0.03: in formula, Y 1 and Y 2 bonded to benzene rings, respectively, are each independently hydrogen or group z represented by general formula or below, the average substitution number of the group z per benzene ring is in the range of 0.2 to 1.0, and n represents integer in the range of 2 to 50: in formula and formula, R 1, R 2, R 3, R 4 and R 5 each independently represent hydrogen atom, C 1-10 alkyl group or C 1-10 hydroxyalkyl group, and A - represents hydroxyl ion or acid ion.