To provide a holder for an inside diameter machining tool which does not impair any discharge of chips when machining the inside diameter, or any feed of cutting fluid.

A fitting recess 38 with a shaft-like inside diameter machining tool with a cutting blade 16 formed on a tip thereof fitted is formed along the longitudinal direction of a holder body 31 at a tip part of the holder body 31 extending in a columnar manner, a slit 44 which is opened in the fitting recess 38 and extended in a plane is formed in the longitudinal direction at the tip part of the holder body 31, a flat plate-like flexible part 33 which is elastically deformed in the expanding direction of the slit 44 is formed, and a pressing means 45 for pressing the flexible part 33 in the direction perpendicular to the slit 44 is provided.