A tool radius adjusting method in a machine tool which comprises: a boring holder main body (10, 20, 50) provided with a housing and a movable body supported on the housing to be slidable relative to the housing in a direction to cross a rotational axis direction and having a reference portion; a cutting blade attached to the movable body; a tool spindle rotatably supported on a spindle head and holding the housing; a position adjusting reference member provided to be changeable in relative position to the tool spindle and provided to be contactable with the reference portion of the movable body; and drive means used for positioning the spindle head relative to a workpiece in machining the workpiece with the cutting blade; the method comprising: a contact step of sliding the movable body relative to the housing so that the position of the cutting blade comes to a predetermined position in a direction to go away from the rotational axis, to bring the reference portion of the movable body and the position adjusting reference member into contact and the method being characterized by an adjusting step of, after the contact step, changing the relative position between the tool spindle and the position adjusting reference member by the drive means in a direction to come close each other to adjust the position of the cutting blade relative to the rotational axis. a boring holder main body (10, 20, 50) provided with a housing and a movable body supported on the housing to be slidable relative to the housing in a direction to cross a rotational axis direction and having a reference portion; a cutting blade attached to the movable body; a tool spindle rotatably supported on a spindle head and holding the housing; a position adjusting reference member provided to be changeable in relative position to the tool spindle and provided to be contactable with the reference portion of the movable body; and drive means used for positioning the spindle head relative to a workpiece in machining the workpiece with the cutting blade; the method comprising: a contact step of sliding the movable body relative to the housing so that the position of the cutting blade comes to a predetermined position in a direction to go away from the rotational axis, to bring the reference portion of the movable body and the position adjusting reference member into contact and the method being characterized by an adjusting step of, after the contact step, changing the relative position between the tool spindle and the position adjusting reference member by the drive means in a direction to come close each other to adjust the position of the cutting blade relative to the rotational axis. a contact step of sliding the movable body relative to the housing so that the position of the cutting blade comes to a predetermined position in a direction to go away from the rotational axis, to bring the reference portion of the movable body and the position adjusting reference member into contact and the method being characterized by an adjusting step of, after the contact step, changing the relative position between the tool spindle and the position adjusting reference member by the drive means in a direction to come close each other to adjust the position of the cutting blade relative to the rotational axis.