A tool holder, comprising: a collet chuck main body having a tapered hole formed in its tip end portion so as to be tapered toward an axial rear end; a tapered collet having a tool insertion hole that extends in an axial direction, a main tapered surface that fits in said tapered hole, and a tip end tapered surface positioned on an axial tip end side of said main tapered surface, and tapered toward an axial tip end; a cylindrical lock nut having internal threads that are screwed on external threads formed on an outer periphery of said tip end portion of said collet chuck main body, for reducing a diameter of said tool insertion hole by pressing said tapered collet into said tapered hole; a tip end tapered member having a tapered abutting surface that fits on said tip end tapered surface, said tip end tapered member being pressed toward said axial rear end by said lock nut; said tapered collet further has a ring groove formed in a portion of its outer periphery between said main tapered surface and said tip end tapered surface, and extending in a circumferential direction; said tip end tapered member further has a protrusion that engages with said ring groove; said tool holder further comprises retaining means attached to an inner periphery of said lock nut, for receiving a rear end of said tip end tapered member; characterized in that a plain bearing positioned in a plane perpendicular to an axis direction in which said tip end tapered member is pressed by said lock nut, said lock nut has an inward protruding flange portion that protrudes radially inward and has a first annular flat surface extending perpendicular to the axis direction; said tip end tapered member has an outward protruding rear end portion that protrudes radially outward and has a second annular flat surface extending perpendicularly to the axis direction; and said plain bearing is inserted between said first annular flat surface of said inward protruding flange portion and said second annular flat surface of said outward protruding rear end portion, whereby the tip end tapered member is allowed to be moved in the plane perpendicular to the axis direction and is aligned with the axis of the collet chock main body by the tapered hole. a collet chuck main body having a tapered hole formed in its tip end portion so as to be tapered toward an axial rear end; a tapered collet having a tool insertion hole that extends in an axial direction, a main tapered surface that fits in said tapered hole, and a tip end tapered surface positioned on an axial tip end side of said main tapered surface, and tapered toward an axial tip end; a cylindrical lock nut having internal threads that are screwed on external threads formed on an outer periphery of said tip end portion of said collet chuck main body, for reducing a diameter of said tool insertion hole by pressing said tapered collet into said tapered hole; a tip end tapered member having a tapered abutting surface that fits on said tip end tapered surface, said tip end tapered member being pressed toward said axial rear end by said lock nut; said tapered collet further has a ring groove formed in a portion of its outer periphery between said main tapered surface and said tip end tapered surface, and extending in a circumferential direction; said tip end tapered member further has a protrusion that engages with said ring groove; said tool holder further comprises retaining means attached to an inner periphery of said lock nut, for receiving a rear end of said tip end tapered member; characterized in that a plain bearing positioned in a plane perpendicular to an axis direction in which said tip end tapered member is pressed by said lock nut, said lock nut has an inward protruding flange portion that protrudes radially inward and has a first annular flat surface extending perpendicular to the axis direction; said tip end tapered member has an outward protruding rear end portion that protrudes radially outward and has a second annular flat surface extending perpendicularly to the axis direction; and said plain bearing is inserted between said first annular flat surface of said inward protruding flange portion and said second annular flat surface of said outward protruding rear end portion, whereby the tip end tapered member is allowed to be moved in the plane perpendicular to the axis direction and is aligned with the axis of the collet chock main body by the tapered hole.