PURPOSE: To finish the outer bore of an axis to high precision and with less surface roughness, and enable cost reduction by providing a cutter extended from one end to the other of the internal surface of a cylindrical tool proper in a reamer for an axis to finish the external surface thereof.

CONSTITUTION: Many grooves 12 are formed at equal intervals from one end to the other of the internal surface of a cylindrical tool proper 11, and a cutter 13 is provided in parallel between adjacent grooves 12 and 12. The cutter 13 is so formed that the chamber of a main cutter 14 for actually machining the external surface of an axis and a sub-cutter 15 for machining a finish margin will form a helix angle (α) with an axial line 'T'. And the flank face 17 of a positive flank angle (β), a margin 18 for a releaf angle of 0° on the opposite side of the flank face 17 and primary and secondary flank faces 19 and 20 respectively having positive primary and secondary releaf angles (γ) and (δ) are formed on a cutting part 16. When a reamer so constituted is fixed to a holder for machining, the outer bore of the axis can be machined to high precision and with small surface roughness, and manufacture cost can be reduced.