PURPOSE: To exchange bars without requiring stand-by time by confirming a part second to the last part of a bar while the part second to the last part of the bar is machined and by starting supply motion of a new bar when machining of the last part is started.

CONSTITUTION: Part corresponding to a part 24 second to the last of a bar 10 is confirmed, and after this part 24 second to the last part is cut off, when machining of the last part 25 of the bar 10 is started, a push rod 16 is retreated to a bar reception position, and a new bar 11 is taken out to the bar reception position of a bar supply device 2 while part to be the last part 25 of the bar 10 is machined. Thereafter, the new bar 11 is advanced so that the head end of the new bar 11 is positioned in the neighbouthood of the rear end of the bar 10 which is machined, and after the last part 25 is cut off, the new bar 11 is advanced to a machining position.