PURPOSE: To effectively prevent vibration of a reamer and to increase the roundness of a hole by setting the number of cutting edges of a reamer to an even number, setting different central angles within a specified range of a difference therebetween, and disposing a cutting edge on the opposite side, with the axis of a cutting edge therebetween.

CONSTITUTION: Cutting edges 11 of a reamer are disposed in such a manner that the angles between radial lines connecting the adjacent cutting edges and the axis 0 are 50°, 60°, and 70° in order backward in the rotating direction, and another cutting edge 11 is provided on the opposite side at an angle of 180°, with the axis 0 of the cutting edge 11 therebetween. A designated bite angle is given to the forward end portion of the cutting edge 11. Further, the margin width of the outer peripheral margin connected to the cutting edge 11 is set to 0.1D-1.4mm when the diameter of the cutting edge 11 is let be D. The cutting edge portion of the cutting edge 11 within the range of 15mm-30mm extending from the forward end to the base end side has a back taper, and the diameter of the cutting edge portion on the base end side from the cutting edge portion where the back taper is provided is constant.