There is provided a novel, environmentally friendly primer for use in the pretreatment steps in electroless plating, which can be easily used in fewer steps with a lower cost. A primer for forming a metallic plating film on a substrate by electroless plating, the primer including: a hyperbranched polymer having an ammonium group at a molecular terminal and a weight-average molecular weight of 500 to 5,000,000; and a metal fine particle.