A blade plate 2 includes cutting edges 5 on both sides in a lateral direction Y. The cutting edges 5 extend along a longitudinal direction X. Each cutting edge 5 includes an arcuate distal cutting edge 8 and a main cutting edge 10. The distal cutting edges 8 are continuous via a distal joining portion 7. Each main cutting edge 10 extends from the corresponding distal cutting edge 8 via an intermediate joining portion 9 toward the proximal portion of the blade plate 2 to an cutting edge end portion 11. The width W8 between the distal cutting edges 8 of the cutting edges 5 gradually increases from the distal joining portion 7 toward the intermediate joining portions 9. The width W10 between the main cutting edges 10 of the cutting edges 5 gradually increases from the intermediate joining portions 9 toward the cutting edge ends 11. The width W11 between the cutting edge ends 11 of the cutting edges 5 is 2.0 mm or less.
