A tool holder (100) comprising: an outside section (10) connected and fixed to a main shaft housing; a rotation input section (20) that is gripped by the main shaft and caused to rotate at a first rotation speed r1; a rotation output section (40) that has the rotational force from the rotation input section (20) transmitted thereto and rotates at a second rotation speed r2 that is different from the first rotation speed r1; a slider (60) that is moved in a first direction that intersects the rotational axis (Os) of the main shaft, as a result of the force from the rotation output section (40); a cutting edge (70) that is moved in the first direction by the slider (60) and is rotated by the rotation input section (20); and a detection mechanism (90) for detecting that the cutting edge (70) is at the origin position.