To provide a cutter of a plate material, making a cut surface smooth and performing chamfering simultaneously with cutting.

This cutter of a plate material is provided with a rotary cutter 13 having a number of blades on the outer peripheral part, and a main shaft 12 supporting the rotary cutter 13 to be rotated, wherein in the respective blades 15 of the rotary cutter 13, at least one side cutting edge 15a is inclined from the base toward the tip in the direction of the other side cutting edge 15b, and the main shaft 12 is inclined so that one side cutting edge on the outermost side contacting the plate material W to be cut is orthogonal to the plate material W.