To accomplish tool position adjusting work automatically and highly accurately.

A tool holder 10 has one end fixed to a shank 12 and the other end provided with a housing 16, on which tools 14 such a boring bar can be fitted, a driving mechanism 18 giving torque to the shaft rotation axially along the shank 12, and a torque converting mechanism 20 capable of radially adjusting the position of the tool 14 by converting the torque only in a single rectilineal among the radial directions of the shank 12.