To efficiently obtain secondary coppered wiring boards of small area from primary coppered wiring boards.

A primary superposed board (A) is formed by superposing a plurality of sheets of the primary coppered wiring boards (1-1) made by fixing copper foils (3a, 3b) to both surfaces of a core plate (2). The primary superposed board (A) is cut into a plurality of boards by a rotary cutter (7) to form a secondary superposed board (B) superposed with a plurality of sheets of the secondary coppered wiring boards (1-2) of small area. A rotary brush having a flexible brush material is provided on an outer periphery. By grinding a cutting end face of the secondary superposed substrate (B) by the rotary brush, each of the secondary coppered wiring boards (1-2) are separated into an upper side and a lower side.