It is an object of the present invention to provide an electroless palladium/gold plating process that can form a palladium/gold plating film selectively only on the copper without generating abnormal palladium precipitation even in a compact sized single electrode or a wiring with a narrow L/S. In order to solve the above-described problem, the electroless palladium/gold plating process comprises: a step (S4) of carrying out a copper surface electric potential adjusting treatment by immersing the insulating base material with a surface on which the copper has been disposed in a sulfur-containing aqueous solution containing one or more sulfur compounds selected from the group consisting of a thiosulfuric acid salt and a thiol; a step (S5) of carrying out an electroless palladium plating treatment on the insulating base material in which the surface electric potential of the copper has been adjusted to form a palladium plating film on the copper; and a step (S6) of carrying out an electroless gold plating treatment on the insulating base material in which the palladium plating film has been formed on the copper to form a gold plating film on the palladium plating film.