Disclosed is a coating agent characterized by comprising: a core-shell-type resin particles (A) each having a shell layer and a core layer; a compound (B) having at least one selected from the group consisting of a hydrolysable silyl group and a silanol group and also having an epoxy group; and an aqueous medium, wherein the shell layer comprises (a1) a urethane resin with 2000 to 5500 mmol/kg of an aliphatic cyclic structure and at least one selected from the group consisting of a carboxyl group and a carboxylate group and the core layer comprises (a2) a vinyl polymer having a basic nitrogen-atom-containing group. Further disclosed are a coating agent for a plastic base material and a coating agent for a metallic base material.