A metal surface treatment agent and a metal surface treatment which impart excellent electrodeposition coatability (smoothness and throwing power) to a metallic base while heightening corrosion resistance. The metal surface treatment agent that is used in a pretreatment of an electrodeposition coating of a metallic base and includes one or more zirconium, titanium, or hafnium metallic elements, one or more silane coupling agents, hydrolysates thereof, or polymers thereof, and an electrophilic reactive group-containing compound (C) containing a homocycle and a heterocycle in one molecule and has a water solubility at 20° C. of 0.2 to 30 g/L, and the contents of the metallic elements, the coupling agents, and the electrophilic reactive group-containing compound satisfy a given relationship.