At a contact step in a tool radius adjusting method, a reference portion of a movable body provided radially movably on a housing of a boring tool and a position adjusting reference member fixed on a machine tool are brought into contact by sliding the movable body relative to the housing so that the position of a cutting blade comes to a predetermined position in a direction to go away from a rotational axis. Then, at an adjusting step, the position of the cutting blade relative to the rotational axis is adjusted by changing the relative position between a tool spindle holding the boring tool and the position adjusting reference member in a direction to come close to each other by drive mechanisms of the machine tool used for positioning a spindle head with the tool spindle relative to a workpiece in machining the workpiece with the cutting blade.