To enhance a shearing effect by arranging one serrations on one side of a cutting surface, and arranging another set of serrations with one set of serrations on the other side of the cutting surface.

A first set of serrations 50 on one side of a channel 48 are not arranged with a second set of serrations 52 on the other side of the channel. Rather, the first set of serrations 50 and the second set of serrations 52 are different from each other in a phase of 180°. A crest 54 first engages with a sheet 26 by arranging the positionally dislocated serrations 50 and 52. Next, since they are driven in a corrugated board along a blade arranged by a valley 56, they can be easily penetrated and passed through the corrugagted board sheet 26. The respective serrations 50 are separated by the serrations 52 positioned on the opposite side of the channel 48 in a border line of a side surface.