To avoid lowering of holding rigidity caused by a fitting clearance and to carry out double constraint with comparatively low working precision in the case of constituting a fitting part against a main spindle of a double constraint type tool holder of a plural number of members.

A fitting projected part 60 of this tool holder 10 is made hollow, and a plural number of grooves 76 a cross-section shape of which is semi-circular and extending along the axial direction are formed with equal angular intervals on an inside of a peripheral wall of the fitting projected part 60. A thin groove bottom 78 and a thick bank part 80 are alternately formed on the peripheral wall. Consequently, it is possible to realize double constraint by absorbing a dimensional error of the fitting projected part 60 and a fitting hole 18 by making the fitting projected part 60 easy to deform in the peripheral direction while avoiding lowering of flexural rigidity of the fitting projected part 60.