To provide a cutter which can excellently cut a metallic product such as a metallic plate and a metallic cylindrical body, particularly, the metallic cylindrical body without practically causing deformation, and a cutting method to cut a metallic quadrangular cylindrical body by using this cutter.

In a cutter blade 2, a sharp cutting tip 21 arranged on the front almost central lower end and a cutting edge 22 expanded upward in a curved shape on both sides from this cutting tip 21, are formed almost in parallel in the thickness direction. A cutter stand 3 is composed of an installing part 31, a groove 32 having a width almost equal to a thickess of the cutter blade 2 and a fixed blade 33 arranged on both sides of this groove 32, and the cutter blade 2 can freely go in and out between the fixed blade 33.