An ID tag (27) is attached to a bottom surface (31) of a hollow portion (17) of a tool holder (11) having a hollow taper shank. Concretely, a circular recess (33) is formed on bottom surface (31) of hollow portion (17), and ID tag (27) is attached in recess (33) by means of adhesive or the like. The center of circular recess (33) coincides with a rotation axis of tool holder (11).