A method for treating the surface of a zinc-aluminum-magnesium alloy-plated steel sheet with a metal surface treatment agent, comprising: a step of forming a zinc-aluminum-magnesium alloy-plating layer on the surface of a steel sheet, and a step of treating the surface of the plating layer with a metal surface treatment agent subsequently after the step of forming the plating layer, wherein the zinc-aluminum-magnesium alloy plating layer is a plating layer containing Al: 1.0 to 10 mass% and Mg: 1.0 to 10 mass% with the balance of Zn and inevitable impurities, the metal surface treatment agent contains a compound having a zirconyl ([Zr=O] 2+) structure, a vanadium compound, a titanium fluorocomplex compound, an organic phosphorus compound (Da) containing a phosphoric acid group and/or a phosphonic acid group, an inorganic phosphorus compound (Db), an aqueous acrylic resin, and an oxazoline group-containing polymer as a curing agent, the acid value of the aqueous acrylic resin is 300 mg KOH/g or more, the content of the aqueous acrylic resin relative to the metal surface treatment agent is 100 ppm to 30,000 ppm as the concentration of the resin solid content therein, the content of the oxazoline group-containing polymer relative to the metal surface treatment agent is 50 ppm to 5,000 ppm as the concentration of the solid content therein, and the ratio by mass of the total mass, in terms of the metal elements therein, of the compound having a zirconyl ([Zr=O] 2+) structure, the vanadium compound and the titanium fluorocomplex compound to the solid content of the aqueous acrylic resin and the oxazoline group-containing polymer, (A+B+C)/(E+F) = 10/1 to 1/1, and the pH of the metal surface treatment agent is 3 to 6. a step of forming a zinc-aluminum-magnesium alloy-plating layer on the surface of a steel sheet, and a step of treating the surface of the plating layer with a metal surface treatment agent subsequently after the step of forming the plating layer, wherein the zinc-aluminum-magnesium alloy plating layer is a plating layer containing Al: 1.0 to 10 mass% and Mg: 1.0 to 10 mass% with the balance of Zn and inevitable impurities, the metal surface treatment agent contains a compound having a zirconyl ([Zr=O] 2+) structure, a vanadium compound, a titanium fluorocomplex compound, an organic phosphorus compound (Da) containing a phosphoric acid group and/or a phosphonic acid group, an inorganic phosphorus compound (Db), an aqueous acrylic resin, and an oxazoline group-containing polymer as a curing agent, the acid value of the aqueous acrylic resin is 300 mg KOH/g or more, the content of the aqueous acrylic resin relative to the metal surface treatment agent is 100 ppm to 30,000 ppm as the concentration of the resin solid content therein, the content of the oxazoline group-containing polymer relative to the metal surface treatment agent is 50 ppm to 5,000 ppm as the concentration of the solid content therein, and the ratio by mass of the total mass, in terms of the metal elements therein, of the compound having a zirconyl ([Zr=O] 2+) structure, the vanadium compound and the titanium fluorocomplex compound to the solid content of the aqueous acrylic resin and the oxazoline group-containing polymer, (A+B+C)/(E+F) = 10/1 to 1/1, and the pH of the metal surface treatment agent is 3 to 6.