PURPOSE: To make the end part in the width direction conform with the external form of a plank, by forming one of both side parts of the end in the circumferential direction of a roll frame-like cutter of a slitter, and the other side, like a wave conforming with the external form of a plank, and like a wave conforming with the shape of both side parts of the end in the circumferential direction of said cutter, and making a band steel base material between both these cutters.

CONSTITUTION: Both side parts of the end in the circumferential direction of roll frame-like cutters 3'a, and 3'b of one side of a slitter are formed like a wave nearly conforming with the external form of a plank 5, and both side parts of the end in the circumferential direction of the other frame-like cutters 4'a, and 4'b are formed like a wave conforming with the shape of both side parts of the end in the circumferential direction. When band steel 2' is made to pass through between both these cutters, the end part in the width direction nearly conforms with the external form of the plank 5.