A boring cutter body (5) is inserted into a circular hole (3) and is rotated and moved in an axial direction, whereby screw-shaped groove parts (11) are formed by using a tool bit (9) provided on a tip end of an outer peripheral portion, and broken surfaces (15) are formed by breaking tips of ridge parts generated by formation of the groove parts (11). Moreover, the groove parts (11) are processed through electric discharge machining using a wire electrode (17) provided to the boring cutter body (5) behind the tool bit (9) in the rotating direction, thereby forming finely roughened portions (41) having more finely roughened shapes than the broken surfaces (15).