To bore such large-diameter holes with portable tools as would otherwise need large machine tools of a floor installation type.

This existing-hole reaming tool comprises a rotary shaft 10, a cutting edge 13 fixed on the periphery of the rotary shaft 10, bearings 12 for supporting the rotary shaft 10 so that it can move axially, and a frame 11 for holding the bearings 12. The existing-hole reaming method comprises a step of positioning the rotary shaft 10 with respect to an existing hole 1 bored in a workpiece 9, a step of mounting the frame 11 on the workpiece 9 with a vise 7, and a step of rotating the rotary shaft 10 with a power drill 8 connected thereto and moving the rotary shaft 10 axially with the tip of the cutting edge 13 along the existing hole 1 to thus cut the internal surface of the existing hole 1.