PURPOSE: To provide a tool and a tool holder capable of substantially avoiding the cross section from being made fragile even if a shank is provided with entrainment grooves and a locking groove.

CONSTITUTION: A tool shank has at least three entrainment grooves 13, 14, 15 different in cross sectional area, and at least one locking groove 12. The locking groove 12 is closed at its end adjacent to the free end of the shank to prevent fallout of the tool when it is engaged by a locking element in the tool holder. To reduce the wear of the tool shank and the tool holder, the entrainment grooves 13, 14, 15 are arranged in sequence in the order of decreasing the sectional area, that is, from the largest to the smallest sectional area, and the locking groove 12 is disposed between the entrainment groove 13 and the entrainment groove 15. The entrainment projection in the tool holder is disposed corresponding thereto.