PURPOSE: To electrochemically remove a burr and a protruding part which have been produced at an electroplating operation by a method wherein a board is treated with acid, the electroplating operation is executed in a process to form a conductor inside a through hole and, after that, an electrolytic polishing operation is executed in a continuous production line while the board is attached to a jig.

CONSTITUTION: A hole is made in a board 1 on which copper foils 2 have been pasted; it is assumed that a hole-making burr 4 produced at this time could not be removed completely. Then, the board 1 is put into a panel plating line; an electroless copper- plated layer 5 is formed; it is assumed that a foreign body 6 of, e.g. a copper powder or the like has adhered to the surface of the copper foil 2. In addition, an electroplating operation is executed in the same production line; an electrically copper-plated layer 7 is formed. Parts of the hole-making burr 4 and the foreign body 6 are transformed into protruding parts after the electroplating operation. Then, an electrolytic polishing operation is executed in the same production line; an H3PO4-H2SO4-based electrolytic bath is used as an electrolytic polishing liquid. Thereby, it is possible to obtain the electrically copper-plated layer 7 whose protruding parts have been removed and which is smooth.