PROBLEM TO BE SOLVED: To provide a new treatment agent, in a plating treatment method including an etching step with a chromic acid-sulfuric acid mixed liquid, which has a stable neutralization action to the chromic acid comprised in the etching treatment liquid, and also can exhibit excellent catalyst sticking capacity to various resin moldings. SOLUTION: The post-treatment agent for etching treatment with a chromium acid-sulfuric acid mixed liquid is composed of an aqueous solution comprising a diamine compound or a condensation polymer of the diamine compound and epichlorhydrin as an effective component(s). In the plating method to a resin molding, a resin molding is subjected to etching treatment using a chromic acid-sulfuric acid mixed liquid, thereafter, the resin molding is brought into contact with the above post-treatment agent, subsequently, catalyst application and electroless plating are performed, and further, electroplating is performed as necessary. COPYRIGHT: (C)2009,JPO&INPIT