To provide a diamond cutter in which replacement work of a cutter body can be easily executed without the deviation of the axial center of a electric tool to a spindle.

The diamond cutter, which is freely attachably/detachably mounted to the spindle 1 with a screw of the electric tool, is provided with a flange body 6 having a screw hole part 2, which is externally fitted to the spindle 1 with a screw and into which the screw is screwed, an outer flange part 4 having a plurality of mounting screw holes 3, and a cylindrical fitting protrusion 5 protruded from the outer flange part 4, and a flat plate-like cutter body 11 having a center hole 7, which is fitted to the cylindrical fitting protrusion 5 of the flange body 6, a tapered through-hole 9, into which a countersunk screw 8 screwed to the mounting screw hole 3 of the outer flange part 4 is inserted, and a diamond cutting blade part 10 on the outer periphery.