A polygon mirror is produced using a diamond tool. The diamond tool includes a major cutting edge and an end cutting edge. The major cutting edge is formed straight by a rake face and a side flank, the major cutting edge having a first side and a second side. The end cutting edge is formed straight by the rake face and a front flank, the end cutting edge having a third side. A microscopic cutting edge is arranged between the second side and the third side so that the major cutting edge and the microscopic cutting edge together create a first corner at the second side, and the end cutting edge and the microscopic cutting edge create a second corner at the third side. The rake face is inclined downward from the second corner to the first side. The microscopic cutting edge is between 0.1 mm and 0.4 mm in a direction of the end cutting edge. The microscopic cutting edge at the first corner is swept back by an amount between 0.21 .mu.m and 0.6 .mu.m from an extended line of the end cutting edge in a direction perpendicular to the extended line. The material being operated on by the diamond cutting tool is held and fed relatively to the diamond tool so that the material is cut with the major cutting edge, is thereafter microscopically cut with the microscope cutting edge, and is thereafter burnish-finished with the end cutting edge.