To prevent the damage of a through-hole and a correcting rod and to form a prescribed size of the through-hole by arranging groove parts recessed in plural positions of the outer peripheral surface of the correcting rod, at the time of expanding the through-hole by inserting the correcting rod made of a sintered body into the through-hole made of a sintered body.

The sintered body 1 arranging the through-hole 2 having the round cross section of the inner diameter D1 and the length L1, is set on a supporting table 3 with a fixed tool 5. Into this through-hole 2, the correcting rod 7 made of the sintered body of the outer diameter D2 having larger by a little than the inner diameter D1 of the through-hole, is inserted to execute after-working to the through-hole having the prescribed size. At this time, on the outer peripheral surface 7a of the correcting rod 7, plural groove parts 8 recessed at about 0.5mm depth from the outer peripheral surface 7a, are arranged so as to make around of the outer peripheral surface 7a, and chamfers 9 are formed on the ridge parts between the side surfaces of the groove part 8 and the outer peripheral surface 7a. Further, the length L3 of the groove part 8 is made to about 1/3 of the length L1 of the through-hole 2 and each interval L2 of the groove parts 8 is made to about 2/3 of the length L1. By this constitution, the through-hole 2 can easily and surely be formed to be prescribed size and the cost of the after-working can be reduced.