Abstract: One object of the present invention is to increase stability for fixing a cutting tool with a holder formed of a sleeve, and to improve machining accuracy of a boring tool using the holder and prevent chipping of a cutting edge. In a holder for a boring tool holding a bar-shaped cutting tool by receiving it in a center hole in a sleeve and clamping it with clamp means, the sleeve is provided with a slit cutting a region from the center hole in the sleeve to an outer periphery surface thereof lengthwise from a front end to a longitudinal partway portion of the sleeve, and is further provided with a fastener tightening a slit-formed portion of the sleeve to change an inner diameter of the center hole.