A method for activating the surface of a base material and an apparatus thereof, which is suited to be utilized for pretreatment in electrochemical treatment such as, for example, electroplating or the like, in which the surface of a base material such as metal can be subjected to degreasing treatment and oxide film removing treatment simultaneously, efficiently and rationally, in which productivity can be enhanced and the equipment cost can be reduced, and in which a waste solution can be rationalized so that the solution can be reutilized and the environmental pollution can be prevented. A method for activating the surface of a base material in which the surface of a member to be treated is subjected to degreasing treatment or oxide film removing treatment. Pressurized carbon dioxide is dissolved in a predetermined quantity of water, thereby preparing an oxide film removing solution having a predetermined acidic concentration.