A surface treatment agent and a surface treatment method capable of imparting excellent corrosion resistance while enhancing adhesion between a surface of a metal substrate and a resin-containing layer. The surface treatment agent for treating a surface of a metal substrate includes an acrylic resin having a carboxyl group and a hydroxyl group, an oxazoline group-containing compound and metal particles containing at least one of Nb, Ca and Nd. The metal may be in the form of a niobium oxide sol.