An object of the present invention is to provide a primer composition for application between the base film and the adhesive layer of an adhesive tape that can improve the adhesion between the base film and the adhesive layer of the adhesive tape and an adhesive tape prepared by using the primer composition. A primer composition, comprising 100 parts by mass (as solid matter) of a graft polymer of a natural rubber graft-polymerized with 15 to 65 mass % of methyl methacrylate and 25 to 300 parts by mass of a carboxyl group-modified acrylonitrile butadiene rubber.